When you resign while cards are being drawn at the start of the turn, the game often freezes (tested in SP only).
I've also witnessed this several times during SP; It always happens during computers turn when he's in the middle of moving characters or using cards.
It has happened to me when I realized I'd brought the wrong party and resigned before the initial cards were dealt.
Happens in MP too: http://forums.cardhunter.com/threads/bug-ragequit-during-block-hangs-battle-engine.6395/