I started the mongod server at the console with -vvv verbose level. On another console I then tried to run a complex map/reduce query. As it took quite a while I killed the query with ctrl-c. As the map/reduce function still seemed to be running on the server I also shut down the server process by pressing ctrl-c.

After that I was unable to access any data in the database. Allthough I could switch to the database, every other command resulted in following error message:

can't map file memory

Also if I tried to run db.repairDatabase() I'll got this error message

{
"assertion" : "can't map file memory",
"errmsg" : "db assertion failure",
"ok" : 0
}

The only thing that helped to bring the database back was to run mongod --repair

Of course I'm aware that pressing ctrl-c is not the correct way of shutting down a server. However, it shouldn't result in such a severe breakdown of service if a not yet finished command gets interrupted.