In Time Attack mode, a boss-like enemy spawned which proved impossible
to destroy: a large missile launcher with one of the blue protection
circles attached, but the protection covered a large circular area
encompassing a large portion of the arena, which blocked both shots and
movement through that area.  This made it impossible to destroy the blue
protection circle, or anything else inside that region.  Furthermore,
the enemy never vanished with time.  Fortunately, it eventually wandered
off outside the walls of the arena and took its protection circle with
it, though not before making my time dip dangerously close to 0.
Another one spawned a while later.

I've occasionally observed enemies (in particular, one of the large
missile launchers earlier in that same game) which seem to get hit by
any shot fired in a large region the instant the shot leaves the ship.
One of those combined with a protection circle that prevented it from
getting hit seems like it would produce this effect.

- Josh Triplett