From Sridhar's email:

This was from the field and I can reproduce it as follows

I can use db.createCollection("system.foobar") to create a collection with the system prefix.

I cannot seem to drop this collection since I get
> db.system.foobar.drop()
Wed Nov 16 16:30:11 uncaught exception: drop failed: {
"ns" : "test.system.foobar",
"errmsg" : "exception: can't drop system ns",
"code" : 12502,
"ok" : 0
}

There is a check in pdfile.cpp in dropNS that checks for this