-Interrupting the gm while they're in the middle of describing a creature/npc that looks dangerous/hostile with the words "I blast it with my rifle" in the hopes of automatically gaining initiative; will usually get your char into far, far, worse a situation then if you waited for her to finish talking.
Additional Ideas (15)
#17 Four ten year olds will quickly turn into an elite adventuring group, given tough enemies to face.
#18 When faking your friend's death, attacking them in broad daylight is NOT the answer.
#19 Doom falls upon all of those who would give the temple of no purpose a purpose.
#20 When the cleric starts the campaign drunk, you know to stack a little more defense into your build.
#21 While the DM is often an evil bastard who is out to kill you, being so paranoid that you take forever to get to the actual battle/trap/danger is just as likely to kill you as whatever trap MIGHT be there.
#22 Just because it is called The Sword of Dragon Slaying doesn't mean it will.
#23 Don't challenge a GMs ruling with a rule. GM is always right and he will prove it to you the hard way if you wish.
#24 Never trust that the dead are dead...even when they are burned and their ashes are scattered to the four winds.
#25 Magic doesn't play favorites. Get out of the way of anybody trying a new spell or testing a new item.
26: The biggest obstacles to online campaigns are timezones, sleep schedules, technical difficulties, and distractions.
#27 There are days when the dice hate the PCs. I just love it when low level weenie monsters slaughter my high level character. Or vice versa being accused of being a killer DM and/or having to save the party from an encounter which should have been a cake walk (yep, the kobold just rolled another natural 20).
#28 Trying to rob someone the DM doesn't want you to will result in certain imprisonment or death.
#29 When descibing unique methods of killing an immortal demi-god like villian NPC in most public places will result in awkward glances, people quickly shuffling away from you, and a strange willingness of bystanders to wait for another elevator (or worse another escalator)
#30 Avoiding the adventure that the DM prepared for will result in some pretty strange twists of fate in which you end up on said adventure anyway.
#31: Saying "Here piggy piggy," to a 12 foot tall troll Lone Star police officer (common in the Shadowrun setting) when you are an unaugmented human and all you have for a weapon is a light pistol will result in the need for surgery to remove said pistol from ones posterior. (Granted the character had the overconfident flaw but there's a fine line between overconfident and stupid.)
#32: Offeirng to tell a dumb blonde joke to all female biker gang is a great way to stop them from immediately beating you senseless.
#33: But will restult in your painful death when, uponn finding out a half dozen of the bikers are blondes and ask if you are *sure* you want to tell the joke, you respond with "Nah, not if I'm going to have to explain it six times."
#34: When exploring a dungeon If the Gm goes out of their way to describe in detail a seemingly harmless rat, mouse, roach, or other common inhabitant; either run away or hit it with your most powerful weapons/spells.
#35 If an entire town is out looking to lynch a suspected murderer... even if you believe he's innocent, when you find him it is best not to go running after him, shouting his name while holding aloft a picture he painted of the victim. He is not likely to believe you have good intentions and might respond poorly.
#36: Giving a paranoid king who's survived multiple assassination attempts a "jack in the box" gag gift that happens to be holding a fake knife covered in red paint will *not* get you reinvited to the annual festival ball.
#37: Nor will implying you are his long lost heir from illicit relations with scullery maids.
#38: When the apparent focus of an adventure is on some routine or otherwise dull task that must be completed, bring along your armor and most powerful weapons, chances are it will be anything but "routine."
#39: Convincing the visiting alien ambassador a latex condom is actually bubble gum will get plenty of laughs from your friends, but not the Admiral or interplanetary space commission when the fun loving ambassador demonstrates his "bubble blowing" skill at the evening banquet.
#40: Borrowing money from one loanshark to repay another one never ends well, especially when the process is repeated a dozen or so times while you try to come up with the original sum. (What began as a mere $10,000 to repair some damaged cyberware grew to well over 100k in short order due to "interest" and "late fees" from each new loan.)
#41: Pretending to join the evil super villians crime syndicate only to tear it apart from the inside only works rarely, and usually will not be successful if you have a current reputation as a "paragon of virtute and justice."
#42: Committing numerous villinaous acts to remove said virtous reputation so you can temporarily join the evil villians organization will likely result in a permanant career (and alignment) change.
#43: When the Gm goes out of their way to describe a sale on heavy weapons, rare monster slaying components, or equally unusual equipment it's best to take advantage of the discount before proceeding with the next adventure.
#44: When a newly encountered alien species asks to speak to your peoples leader, claiming to coincidentally be just that person never ends well.
#45: Jedi mind tricks may get you a free night in the brothel, (or with that pretty twilek dancer at the bar) but will do nothing to prevent or cure the alien STD's you were exposed to.
#46: Or remove suspicion of the jedi council you're the father of said twilek dancers children a few months down the road.
-Picking on angry Shaolin monks for "wearing girly dresses and being a bunch of Buddha belly rubbing pansies," in a modern day horror campaign is a great way to spice up an otherwise sedate investigation scene.
-Doing the former is also a great way to educate yourself on the games multiple combat/and first aid rules in the same scene.
-Key scratching a senator/congressman's car may be a inventive/covert way to make a political statement and "sticking it to the man," but is usually best done when they are not currently inside of it.
Giving a prospective employer a link to Goatse as your Mercanary teams website homepage as a practical joke will usually not get you that hazard pay bonus you were haggling for.
Skunks make great familiars for wizards, not so much for those impersonating them.
-Attempting to use rabid porcupines as catapult fodder, nuff said.
-Swallowing 27 condoms full of stolen gold dust (in an attempt to to avoid giving your team mates an equal cut of the profits after your mercenary op in Africa,) will result in severe internal injury and death when the private plane you are flying on encounters a rough patch of turbulence and the 7+ lbs of metal begin bouncing around your abdominal cavity.
Attempting to draw a super villain out in the open by dressing up as them and attempting to commit random acts of kindness and charity usually results in ones arrest.
In above scenario also a loss of respect by team mates (and the public) if the individual they were impersonating is of the opposite gender.
Live alley cats swung by the tail do not make for effective maces or flails, no matter how drunk and desperate your character is.
Proving someone is not undead by strangling them until they turn blue will usually not result in their gratitude.
Pulling the metal pin of a grenade with your teeth is especially unwise when the temperature is well below freezing and said grenade has been hanging off your harness for the better part of a day in such sub zero temperatures.
Assuming a bag of white powder you snatched from a rival gang members car trunk is cocaine and immediately snorting a line of it to feed your drug habit will have eye opening results when it turns out to be weedkiller.
Using det cord (detonation cord/Primacord used for triggering explosives) as bootlaces and small amounts of C4 hidden in your insoles so you "have a ready means to escape from a locked room" is potentially life saving, but can be just a much of a hindrance when the customs agents bomb sniffing dog falls in love with your footwear.
Black market fences (and chop shops) are usually not interested in construction equipment, or emergency service vehicles.
Especially if the stolen Swat Van still has security personnel quietly hiding in the back.
Mixing up a squeal of excitement, with a squeal of excrement, is a sure fire way to ruin a chance meeting with an old friend.
#65 There is nothing more useful than a 10-foot pole. Ever.
#66 As a Thief, repeatedly poking fun at the Fighter for having a Cha of 3 in exchange for a Str of 18/00 is a quick way to get your nose remodelled.
Jumping off the side of the docks in nearly 100lbs of plate armor to swim after an escaping thief will result in your expedient sinkage and drowning, no matter how good you are at swimming.
In a supers game attempting to subdue a suicide bomber wearing several lbs of c-4 by tazering him with the super power "electric blast" will not end successfully.