I know it’s been around forever, and it’s been raised before, and Memrise evidently is either not interested in solving it, or finds finding a fix mightily difficult (same difference to us users, of course) - but as I see this daily I thought I’d document this with a) a new browser and machine for me, and b) with an interesting addition of Memrise apparently calling Facebook - ?! 
Good idea! If you document anyway, would it then be a good idea to edit the post and add with which course this happens, or with all courses, whether it happens always or only in some specific or random situation and what the OS and browser is? Just a suggestion. But anyway, good that you document this, because I virtually never have with other websites messages like this.
Fair point!
Observed on Opera 54 running on Windows 7
Error occurs towards the end of a general (ie not course specific) reviewing session - sometimes when there are fewer than 10 items left to review, but always at the end, when the button has gone white