In their attempts to stop users sending links to each other, flickr try to spot when you have written a link and stop you submitting. But they are very good at misclassifying non-links too.
My transgression came down to an email address. How shameful that a company of their resources cannot invest in making their software sophisticated enough to say "we do not allow you to send email addresses". Instead, it says "don't include links". An email address isn't a link and the unhelpful error message seems to be catching other fools apart from me.
In my case, the problem was avoided by spelling out "@winster.org" as "at winster dot org" - simple enough but it took me far too long to track it down.