I was adding a feed bot to the French Language & Usage chat room that would feed all questions from Travel Stack Exchange that are about French-speaking locales.
(I'm not 100% sure this particular feed is a good idea, but that's a topic for FL&U Meta, not here.)
I tried making http://travel.stackexchange.com/feeds/tag?sort=newest&tagnames=andorra+or+belgium+or+france+or+french-guiana+or+french-polynesia+or+guadalupe+or+luxembourg+or+martinique+or+mayotte+or+monaco+or+new-caledonia+or+quebec+or+reunion+or+saint-martin+or+switzerland the feed URL, but…
Trimming the URL down a little (no harm done for now, I removed tags that don't exist yet) made it go through. It looks like there's a 250 character limit for the URL.
Please provide a clear error message, and increase the limit.