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Helen, I'm pretty certain that on AIBU they have been splitting the Adsense half and half between cookie-based and page-based ad-serving for the last 3-4 weeks.
I thought it was deliberate A/B testing .... but maybe someone just pressed the wrong button.
That's why people are being ad-stalked ..... once the cookie sniffers at Google pick up on a profitable keyword on a thread (anywhere on MN) it will follow the victim reader on the AIBU threads they read.