I had bolas' citadel in play and a necromancy on top of my library. When I tried to play it at instant speed on my opp's turn the text said "select play Necromancy" or something similar, with citadel highlighted in blue. I clicked citadel, it highlighted orange, and I clicked submit. Then Necromancy moved from the top of my library to "the stack" area of the screen on the left, but it still just glowed blue and didn't actually cast. I could click it again on the stack and get the same interaction, but I couldn't actually cast Necromancy. I ended up having to pass the turn without casting it at instant speed like I wanted it.