Comment 4 for bug 623152

stewart hit this too. Basically this seems to be due to the file being lost after a machine crash:

Some things we could do here:

1- have an option to globally fsync after all file operations, which would at least save people hitting this twice. (bug 343427)
2- report this more cleanly.
3- perhaps have a way to recover, though since the branch has lost its idea of where its tip is, that's not trivial.