Hinges. I've changed the hinges out on 3 doors in the house, no problem. 4th door, for the life of me I can't figure out why the door is sticking. I should've paid more attention to how the previous hinges were installed on the door, as I don't think they were on straight.
Ugh, our old front door was like that. It used to constantly stick, no matter what we did to it. We still have no idea whether it was due to expansion/contraction when the temperature changed, whether it expanded when it got wet (presumably from dew or something, since it happened without rain), because the hinges kept going wonky, or because the entire house was slowly falling sideways, putting pressure on the doorframe, and making it stick. All were very real possibilities, and we never did manage to fix it.
Our new front door is much better, now we've moved house. It's nice not having to rugby tackle the front door in order to get inside any more.