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1.
what people advise you not to do during mildly precarious and trivial situations
Guy 1: Hey, can you hand me that flyswatter over there? I gotta take care of this tiny daddy longlegs crawling up the wall here.

Guy 2: Don't be a hero, man
by Don'tBeAHero May 14, 2010
 
2.
In the world of online gaming, usually used sarcastically, e.g. when running enthusiastically into a high-risk situation where failure is immanent. Sometimes this can actually help the player's team - for example, if the enemy is distracted by the hero - but it usually is done only for amusement.
There is a famous video of Leroy Jenkins being a hero.

While playing Team Fortress 2, I decided to be a hero. I ran around as a civilian, distracting the other team while my teammates captured the control point.
by lamgood October 15, 2008