Definitely I learnt something new today! Thanks for this challenge.
Wow - you turn you back for 1 min, and then @JoeM posts the next challenge while you're not looking, and before you know it there's dozens of bright people who have solved the problem already.
I gotta say, people really swung for the fence on this one with charts, and graphs etc - I just just used some of the analytical tools (violin plot; histogram and a scatter plot) to see if there are any obvious anomalies (and there are :-))
was impressed by @Joe_Mako 's approach - in retrospect it's obvious, but the trick is seeing it the first time.