This tag indicates a reproducible problem on the site that you believe is due to a mistake, malfunction, or programming error.
When reporting a bug, try to describe it in a way that makes it easy for readers reproduce. "I saw some sort of error while I was doing something on one of these sites" doesn't get you very far.
If there's a visual anomaly associated with the bug, then post a screenshot, circling the portion that offends you.
If your bug is only when you're using a beta browser, though, be aware they're not supported. Get a production browser and report back only if it's broken there too.
Interesting bug lists:
See also: feature-request
bug is one of the four mandatory tags for meta sites. The other mandatory tag options are feature-request, support and discussion.
