Guild Wars 2

https://wiki.guildwars2.com/wiki/Fencer's_Finesse

Bugs: This mesmer trait procs on ambient creatures. When the trait activates, it puts you into combat.

How to reproduce: Be a mesmer, select this trait, kill some ambies. Normally, this wouldn’t put you into combat, but it does, and the icon appears on the buff bar.

I don’t think ambient creatures are supposed to affect the core combat too much, so this might also be seen as a balance issue (that said, the trait is absurdly easy to stack to ten in normal combat and each stack barely does anything)

When you attack an ambient, you are in combat. It’s that way for all classes, as far as I know, regardless of any traits.

Ambient creatures have always put characters in combat, under specific circumstances. The ordinary autoattack can put you in combat, if a sigil/rune/trait causes something to happen to ‘hurt’ the creature before it dies, e.g. is it gets poisoned.

If the fact that this happens at all is a bug, it’s an old one.