Explosive environmental objects

Usually these are barrels, often red barrels. If not barrels then they're at least (partially) red. And in some cases they simply have more realistic flammable sign, in red. Luckily some games escape the red-means-shoot-for-more-explosions cliché. According to some reports from game developers, playtesting suggests that people are more likely to actually shoot red things in general (so a green colored exploding thingy is less likely to be utilized than a red colored exploding thingy).

These are like a double-edged sword to many. They're great to have conveniently placed where enemies will appear, but they also spell your doom if the placement is in favor of your opponents.

Commonly these exist in form of oil barrels, compressed gas tanks, or crates filled with explosives.

See also: chain reactions (often blowing up something causes other explosive things nearby to explode as well), minefield, mines