I'm not sure if this is a bug, but I will post it anyway. I think it might be.
During my most recent C&E, a Mudkip was setting up Curses. The Mudkip was up to plus two vs. the opposing Togepi. Both pokemon were level 100, so the minimum-maximum damage range is not very big.
The first time the Mudkip used Waterfall, it did 55 percent at +2. The next turn, the Mudkip got fully paralyzed as the Togepi went for Morning Sun.
Then, the Mudkip used Waterfall again. This time, no crit, it did 95 percent.
I don't know what happened here, but it confused everyone. Unfortunately, I couldn't get a screenshot of this, but trust me that it happened.
EDIT: I got the replay.
http://replay.pokemonshowdown.com/ou-205080172