Repro: Force-push multiple heads to the wiki repository. Then, try to edit the wiki from the website.
Error: A "500 Internal Server Error" page gets thrown up. This doesn't give any feedback about the cause of the problem or how to fix it. Merging the heads (offline) and pushing that resolves the problem. (First encountered with the repository '/aysz88/mortus'.) The displayed content also seemed to be unpredictable/unexpected to the viewers of the wiki, though I wasn't paying attention to that myself.
The quick fix is probably to detect this condition and put up a more informative error message alerting both writers and readers of the condition. I don't know what the 'Right Thing' would be (should it be resolvable from the site? is something already expected to happen instead?).