When an index build as part of a run of mongod --repair fails, the cleanup code that saves aside the last error information, drops indexes, and then restores the original last error value erroneously asserts that there is a last error object. The code is robust to there being no such object, and when run with --repair, there isn't one. In this case, the root cause of the index build failure will not be reported, leaving only a message such as the following (from v2.0.7):

Tue Feb  5 00:22:37 [initandlisten]  dbname Assertion failure le && !saveerrmsg.empty() db/pdfile.cpp 1724

Behavior not confirmed in 2.4 or 2.2, but code is similar to 2.0.