The engineer's mining light was among the first hats ever added to the game. It's so old, that it's still rigged to the engineer's head instead of his helmet. It's also a special case as the light itself is an addon to the base helmet, rather than including a duplicate of the helmet mesh itself and simply replacing the original. This causes it to clip horrendously whenever his helmet moves (which is during a majority of taunts). This mod rigs it to his helmet instead, fixing the issue.

Mod version here: https://gamebanana.com/mods/620582