Sometimes this happens to me with screen recorded footage. I found it useful to render the clip by itself away from other clips. In other words, try to move the clip to the far end of your project and render it alone and then place it back where it belongs and render again.
Hope this helps
If you are speaking of Rendering (creating what is essentially a proxy file for the smoothest possible playback), one can use the WAB (Work Area Bar) to limit the Rendering to just the Clip in question. If you mean "rendering," as it is referred to in other NLE's, where it is Export, one could also use the WAB to just Export that problem Clip, and then Import that output file, as a replacement.
Also, there have been some issues with Mac's not handling "synthetic video" well. That would include Black Video, Transparent Video, and Titles. I do not recall hearing of those problems, since CS5, but they could still be around. In those cases, doing Exports for the problem Clips, and replacing the synthetic video with those output files, almost always fixed the issues, back in CS4.
So we figured it out how to solve it.
The problem was that this problem occurred when implementing it on track num. 30. From track 30 and higher, the crop effect causes this problem. when we took the same titles, in the same sequence and lowered it to, say, track (video track) 25 - everything rendered OK.
Have you ever encoutered such a phenomena?
I just updated to CS6 (6.0.5) and it looks like this "crop effect - failed to export" issue is back, but on every track (well at least on 1, 2, and 3). I just spent the whole day trouble shooting this, and sadly, Premiere's crop effect, was the last variable I could have removed to pin point the issue.
I've got it on a Win7-64 machine with GPU Mercury turned on. The video plays fine off the timeline, but in export and Adobe Media Encoder, the moment the render gets to the first frame of the cropped jpeg, it reports as "Failed" with an "Unknown Error".
My final work around was to crop the images in PS and save. Of course if I had wanted to keyframe the crop, I would have been out of luck, or I would have had to animate-crop in AE if that would work (but who knows).