I seem to recall that in the original Halo, some of the short little Covenant enemies would also sometimes freak out when the player attacked then ducked behind a corner out of sight. It could work to the player's advantage if they hit them with a plasma grenade - the freaked out baddy would sometimes run screaming back to his freinds just as the grenade went off! :P It didn't always work, I think a couple times they came running at me!
I get more annoyed by RTS AI's - even on "Normal" difficulty, they seem to have omniscience of the whole map and can "concentrate" on several objectives (capture point on map, attack enemy base, hold resource mine, build secondary base, etc...) at once, more easily than a human player. In "campaign" modes, they tend to be too heavily scripted - I find this especially true in Supreme Commander: hit one objective and the AI simply waits for the player to make the next move.