I would recommend that no hint of these effects be telegraphed when (or if) the potion is initially tested, or sold. Magic might reveal something mildly off with them, but nothing too significant.
1. When the potion is imbibed, a brilliant, blinding flash of light blinds onlookers momentarily, during which the user becomes invisible.
2. User appears to explode violently into bits of gore, which fade from view after a few seconds. Those close will feel ‘stuff’ hit them, but nothing actually happens. User is unharmed.
3. User begins to convulse and dissolves quickly from view, as if enveloped by a powerful acid. Again, they are unharmed but simply rendered invisible.
4. An illusion of the user simply walks in a random, but reasonable direction. If a precipice is available, they may cast themselves over and fly away. User is still where they started and invisible.
5. User is apparently immolated.
6. User shrinks out of sight. Actually normal sized and invisible.
7. User appears to be drug off by a scaly hand/tentacle/stone arm/beautiful arm into a portal that appears nearby.
8. User vanishes with a loud ‘Pop’ sound, implying teleportation. Still just invisible.
9. User appears to become a cloud of vermin, which disperses in all directions.
10. User appears to gag, changes to an odd color and falls over, apparently dead. Body feels substantial for about 10 seconds afterward and then fades away, presumably giving the user time to leave unseen.
11. Ground opens up and appears to swallow the user.
12. User appears to turn to stone, then crumbles into a pile of sand. User is just invisible, but the sand remains.
If you are cruel, during the first minute or so, the user will be unable to dismiss the invisibility effect so as to perpetuate the illusion that something bad has happened to them. Especially cruel GMs would allow for a off chance these effects are NOT illusory.
Assuming potions of Teleportation, Gaseous form, ghost form or the like are available, these too could also have these effects applied.
Additional Ideas (1)
14-User teleports 10 feet away. Not safe to use in a confined space.
15-User remains invisible until he or she speaks.