So this is a new one. I left my laptop alone with two Gmail's and Google Reader open in Firefox. When I came back they had locked up, as Firefox sometimes does.
I restarted Firefox (kill -9 ftw!) and when I did Google had a quick humanity test for me.
I assume that my Firefox was spamming them with requests while it was locked up, so this isn't "weird" per-say, just new to me. Thought I'd share!