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It was a new listing, there were no other clicks in weeks after, though there were many views of it. The 300% CTR sort of says it all! Contacted Adwords support staff, they say that "flooding" is blocked.... someone else must have clicked also... 3x the number of views in a minute (well it took about 10 minutes to register, but then no other clicks for over a month!)
A little scary - someone could really run up a bill on a competitor with an itchy clicking finger!
Now in all fairness to Google, they may have fixed this, it was over a month ago that I warned them (and they said it wasn't true).
But how else do you get a 300% CTR?