I hate it when I buy tickets online for the sole purpose of being able to pick my seat and I get there and someone's already smurfing sitting in them.

90% of the time they'll move once I tell them they're in our seats, but it's that remaining 10% of people that either
a) try to convince me that I'm wrong
b) say something like "bro, it's fine - just sit somewhere else"
that really cheese me off.

I reserve couples seats for a reason ;_;