This was my easy challenge for today:
'Play quiz Here There Be Monsters and score at least 812 points using Timed Quiz format.'
I played this quiz in timed mode, never having played it before, and got most of the answers correct, but when I finished, I got this message:
'Inconsistent score. Message sent to admin. Aborting.'
And I have not been credited with completing the challenge. This is not the first time this has happened to me.
Mazza

You probably got a hiccup in your computer timer. This happens if your computer corrects its clock (via internet) while you are playing a question. If it corrects back by a second or two (your internal clock was fast) and you answered quickly, it can result in a negative time and an above 100 score for a question.

That's obviously impossible and thus the entire game is voided. As it was only a regular quiz, you should probably be able to replay it.

If this happens to you with some regularity and there is a pattern to it (such as it happening when you play really early, within a few minutes of booting up your PC), simply avoid playing at that time. Normally the time sync should happen within a few minutes of booting and once the system is up, it should check often enough that it won't have to make major corrections to your system clock even if your mainboard's internal clock is off by a bit.