The worst for me is getting back from holidays and having to suffer several additional months of ads for hotels in Japan everywhere on the web, reminding you that you're not there anymore.

That's actually what made me install adblock on my new system even though I was resolved to try to withstand ads this time.

Amazon is the champion of that

"Oh you just bough a mobile case, let me show you ten other types" or "You just bought this book, here's the paperback version of it"