In the powered cube, I have encountered a couple of mana issues around Lurrus and Black Lotus. The client will pass the turn and not allow you to sack your Lotus, replay it, and pay for an expensive spell. For example, Lotus plus Lotus plus three lands is 9 mana, but the client auto passed and did not allow me to cast my 8 drop.

A similar bug occurred during casting Channel early on in the game when the time extensions have not been issued yet. Paying for the mana for a large spell with Channel when you have other sources of mana (like from Lotus) leads to easily roping because you have to click for channel each time to avoid the client spending the mana wrong. This cost me a match because I lost the life and did not get to use the mana.