The *bug command acts as a sort of safety net for your code under specific conditions, and allows you to display a custom error message should a player manage to reach a part of the code they shouldn't.
*if (bullets_in_gun < 3) You have less than half a magazine left! Be careful! *if (bullets_in_gun < 7) You have a full magazine. Mow 'em down! *bug Oops, something went wrong! Please report this as a bug.In the above example, if for some reason the variable "bullets_in_gun" exceeds 7 the game would have continued with whatever code followed this section, but likely would have ended up in a game stopping error later, leaving you clueless as to the cause. With the *bug command in place, your chosen message will be thrown in error instead, immediately alerting you, your testers or your players to the inconsistency.
Looking for a little more detail? Try the wiki page