If you think you have immunity from something, make sure you really have immunity from it before tempting fate. Like if you have a ring that you think makes you immune to fire damage, stick a candle under you hand before you try swimming across lava.

From the other side, this is a cool limitation you can put on protective items. Sure, a Ring of Fire Resistance will let you get away unscathed from sticking your hand in a forge, or standing at point blank when a fireball spell goes off. But who says it'll work just as well for dragon breath, after you walk out confidently in front of a huge angry red dragon and taunt it with insults about its mother?

Transcript

R2-D2: All right, let the expert take over opening this door.
[SFX]: < bedoop whir dit boodit-ting-pow >
R2-D2: I plug into the data socket.
Han: I take cover behind R2!
R2-D2: Sure, they can't hurt me anyway.
[SFX]: < whir doing boop-poppy doop >
GM: Pete: Take this note.
R2-D2: Ooh. Wow, okay. <roll> 3.
GM: A trooper shoots R2.
[SFX]: Pow!
[SFX]: KABOOM!
R2-D2: I thought they couldn't hit live targets!
Leia: They can't hit organic beings, you metal moron.
[SFX]: kzzzzt... zzzzt...
R2-D2: Stupid meatbags.
Han: Wow, there goes my cover.
Chewbacca: You realise that if you had covered R2, neither of you would have been shot.