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. Clearly state what you did to discover the bug, the steps required to reproduce it, the resulting system output, and the expected system output. "I saw some sort of error while I was doing something on one of these sites" doesn't get you very far.
If you're reporting a bug that seems to have existed for a while, be sure to search the site to make sure it hasn't already been reported; if it already has and you also ask your question, yours will be closed as a duplicate. Follow the instructions at How do I get attention for old, unfixed bug reports and feature requests without official responses here on Meta? to get attention for the original report. (An exception: if an existing bug has already been marked as "completed" or fixed, but seems to still be taking place, go ahead and file another bug report as it could be a regression; the team prefers that users file new bug reports rather than removing the "completed" mark on an older one.)
If there's a visual anomaly associated with the bug, then post a screenshot, circling the portion that offends you. Applying a tasteful drop-shadow to your screenshot is optional; drawing the circles freehand is encouraged.
If you can't get it to happen consistently, try harder; if you still can't get it to happen consistently, be sure to mention that before we all waste our time trying to reproduce this
awesome new feature bug you've found. Bug reports that can't be adequately reproduced by the community may be closed as no longer reproducible.
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.