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. Applying a tasteful drop-shadow to your screenshot is optional; drawing the circles freehand is mandatory.
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.
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.