Yet when i have 7 cards in my hand it will say i have 8 and must discard one, the missing card isn't an option to discard.
This happened to me twice yesterday, both in the arena.
I'm not sure what caused it or how to recreate it, but i did have my window restored down using firefox both times it happened, and it has never happened when i had it maximised as i almost always do.
Sorry for the lack of evidence but i didn't happen to be recording my screen when it happened. Which is also ironic because moments before i was in fact recording. (I felt like recording because i got an awesome first hand and like a 6 turn finish, but decided against saving it, knowing i had no one to show it to
)