Couldn't cast a UU creature with UU available.
I was trying to cast Merfolk Trickster on my opponents end step and for some reason I couldn't cast it.
Kept prompting me like I had no lands to tap for it, but I had 1 island and 1 castle vantress untapped.
This was on the final turn of my game before my opponent conceded to me. I guess he realized I already had lethal on the board.
These issues seem to present more often when I have connectivity issues.