Trinisphere + X Cost Artifacts
I had Trinisphere in play and the client was not allowing me to cast Chalice of the Void for X =1 and pay the aditional cost. It kept just allowing 'Cancel' or 'Adjust' and Never 'Done'. It would only allow adjustments to X = 2 or greater.
This is a pretty major issue for blood moon/prison/stompy decks. Chalice on 1 required 4 mana to cast, until I was able to add the over-charge the notice just said, "click Done when to finish paying" with no "Done" button available. Paying 4 and adjusting X to 1 summoned the "Done" button though.