One of the many, many oddities of Opera.
Every browser has just as many oddities as any other browser, all that matters is which browsers you actually test on. And I say this as a web developer myself. Don't test on Firefox, and it will not work on Firefox.
OK, to be more constructive, here's how to reproduce it: Take the latest Opera (10.10) and just type anything from the start to the end. Write a title of a normal length, add some text, and the type several tags, possibly using the auto-complete feature. And hit the "Post Your Question" button. For me it fails in 99.99% of the cases.
However one must note that copy-pasting a bunch of tags works. Or perhaps it works if you hit "Ask Your Question" before the auto-complete kicks in.