I don't buy online that often, and so far only really from Amazon where such things don't generally happen. Although... since Dom mentioned the Monaldi & Sorti books last month, I ended up looking if they had been translated into English yet (what with the French translation coming out only now, a number of years after the Dutch version), and ended up on a page on amazon.com (or was it amazon.co.uk? either way) selling the book. It clearly mentioned the language was German, but evidently some people don't check that closely, or assume that the first hit they find for a book will surely be in English on the American/British Amazon (the titles of all Monaldi & Sorti's books are single Latin words, so the title didn't really help any). So the book had several one-star reviews complaining it was in German instead of English.

Anyway, so no, haven't had that happen to me, but I can imagine it must be annoying. I wish they made more bilingual versions, in general... you can always just buy a translation, if the book is not too obscure, but it's such a pain to have to have two books open side by side.