This bug happened on Twisted Treeline. It could also work on other maps, but I haven't tried it yet.
What I did: used Q on Nexus on twisted treeline
what I expected: either to deal increased damage, as stated on Q's tooltip, or attack it with Q not activating on Nexus and deal normal aa damage
what happened: Garen just refused to attack the Nexus at all. for a few seconds he'd just stand near it until Q went on cooldown.
I was using desert trooper skin, but I don't think it's skin related. I think it's a big drawback for Garen as his Q is great for taking turrets, inhibitors and nexus.