This will happen at times if you've been idle for too long so it auto logs you off and you have to either reload the page and login again or sometimes just reloading will set it straight.
I have it on my laptop every now and then as I do leave it sitting out but never got it on my phone.
Well I just got it again less than a minute after logging in. Sure it will go away but its not exactly ideal for someone who's just joined and might get that popping up. First experiences count for many people. For some bizarre reason posting on this board seems fine and not others.
Incidentally, this is exactly the message I was getting back when I tried to enter a password with non-alphanumeric numbers before. I think it may be partly to do with the website getting what it thinks is unexpected input?