The Top singleton object keeps state about find/update/remove operations against non-existent collections. This is unexpected.
If a malfunctioning or malicious client repeatedly issues queries against unique non-existent collections, the "top" command will start to return an error after the size of the response object exceeds the maximum size of a BSON object, and the Top usage map will grow to an arbitrary size in memory. This memory can be reclaimed with a restart of the mongod process.
Reproduce with the following script: