HS 2 , "infinite duels" (haven't tried otherwise).

I was having trouble using hotkeys to give myself and the AI cards and permanents, so I marked all the phases to stop at them.

If the Draw phase is marked to always stop at it, the game will hang indefinitely until you de-mark it. It doesn't happen all the time, but I haven't been able to spot what exactly sets it off. Prompt comes up, clicking on it does nothing, and the card-being-drawn animation hangs (you see a cardback in your hand). This doesn't seem to happen for the AI, only for the player (or so it seems). Figured it out - if the phase is marked it doesn't automatically draw the card, you have to click on the cardback for it to get drawn and then things go on normally.

If the Combat Phase is marked always to stop, it looks like there's an empty attack launched, but it looks like it's happening in the second main phase rather than the combat phase. This might be a visual thing - if I have no creatures and an "empty" attack happens I can see the phase indicator light move to the post-combat main phase, but if I have a creature out the combat phases bar is overlaid over the regular phase bar so I can't see the phase icons, and when the combat bar goes away the marker is at the post combat main phase.