Did it twice to confirm, but slightly different.
If you have a lvl'd merc (amazon - blitzer - mighty blow vs AI Chaos) and you blitz or block with follow. The action doesn't complete and the game becomes unresponsive. Every piece is still animated, can still click around, but cannot queue another action, and the game cannot progress, also clock is stopped.
I have not tried this in multiplayer.
Edit: also cannot concede at this point, only option is reset
Edit 2: by lvl'd merc, I mean purchased level