Playing on iOS. An Instant spell was cast and opponent initiated and cancelled countering, but rather than continuing to resolve the Instant spell was stuck in the sidebar unable to resolve. It continued to be highlighted as if playable but I was unable to further interact with or remove it. Apart from never having done its job, it was annoying to have to pass everytime my opponent did anything when there was nothing I could do to remove the card. As soon as the match was over the error was fixed and the card moved back to my hand. If I am mistaken and this was a normal part of gameplay, there was insufficient help text to clarify what needed to be done to get the card out of the sidebar.