I don't really know how to replicate it, but when I spectate someone playing in the game mod other than the one I have currently set, I sometimes get the wrong highscores shown. Usually it's mania highscore, which makes the spectated player to easily take a lead... even if he don't really do this 
there's more to changing modes, but it's already reported, so I will only mention this (not sure if it wasn't already reported too ^_^)
https://osu.ppy.sh/ss/4411204
there's more to changing modes, but it's already reported, so I will only mention this (not sure if it wasn't already reported too ^_^)https://osu.ppy.sh/ss/4411204