Combustible (3.5e Flaw)
From D&D Wiki
In short, when the material that you are comprised is exposed to a very high temperature it violently combusts in the form of a dramatic explosion. You have learned to like rather cool places as a result.
Effect: If the PC touches flame for longer than one round (including magical flame), or is damaged by a powerful shock (dragon's breath, call lightning, etc. - any shock dealing capable of dealing 2d6 damage or more) you burst into flames. If these flames persist for more rounds than your Constitution modifier, you violently explode, dealing 10d6 damage to yourself and everything in a 15 foot sphere around you. Also, you may not ever gain fire resistance or immunity.
Roleplaying Ideas: "What a lovely day, the sun is shining, my friend is grilling, I'm leaning on the grill top," It was at this moment you remembered your horrible secret... you are highly and very deathly allergic to third degree burns...
- see the lovely graphic (shown right) about what happens next!