Over the last few days I've been testing a few WordPress themes that use TimThumb for generating thumbnails of both images and video.
TimThumb works very well in most situations, though you can run into the odd issue if your directory permissions aren't correct. In order for it to work it has to have somewhere it can store the thumbnail images.. and that has to be writeable by the server.
However, unless the server you are on has incredibly tight security settings, it's quite easy to "fix" that problem.
But I was still seeing issues on one site where it simply refused to display any thumbnails.
To add to the frustration there were no errors in the logs.
After pulling out some of my hair (I don't have a lot to begin with) I finally realised that the issue was with Zemanta's invisible pixel image.
Removing it from the posts resolved the issue.