That and the time PBP takes. In both cases where I argued with the DM (to my eternal shame!) it was where I knew about the thing in the days/weeks prior, but didn't think of it during the moments I took to post. If we'd been sitting down at a table, it's possible that I would have forgotten them in the seconds of my turn, maybe, (I can't say I always remember everything at the table, naturally!) but I had them both in mind outside of the time when it became important, which is its own frustration.I think its more a factor of we aren't playing 20th level characters "organically". We are just putting some characters together and going to work. In a normal campaign, the players have grown into these characters (and the DM has watched them grow) over many many sessions, so the abilities become ingrained. Under that condition its a lot easier to run 20th level games, because everyone just knows what everyone in the group can do.