If it's on some obscure bug tracker that I don't have an account for then the odds of me reporting it are almost zero because chances are there's an alternative to what your software does if it happens to be a show stopper and most of the time if it's a wide spread project there will be fixes to the bug in other places.
It's annoying to make an account, go through all of that stuff and then realize most of the time nothing happens with your report.
1. Is it a giant faceless corporation (e.g. Microsoft, EA, Adobe)? If so, ignore. 2. Is it difficult to reproduce? If so, ignore. 3. Is it so subtle that it isn't really worth my time to track things down? If so, ignore. 4. Is it so amazingly obvious and disruptive that, unless the bug only affects me, they've obviously heard an earful from everyone else who's seen it? If so, ignore. 5. Do they have a twitter account that actually appears to read and respond to people? If not, ignore. 6. Tweet at them and ask them what to do about it.