The "staticYield can't unlock" message is printed to the diagnostic log when a a query that cannot yield locks is running for a long time. As of 2.6.0, examples of queries that cannot yield locks include (partial list): findAndModify operations with a sort, queries run inside the "eval" command.
Example message logged by findAndModify with a sort:
It is often the case that these queries are running for a long time because they are unindexed or poorly indexed. In this case, the problem goes away once an appropriate index is created.
However, this message is logged too frequently; it should only be logged periodically. Reducing the frequency that this message is logged will help eliminate log spam for users that don't care that they have long-running queries that cannot yield locks.
This message was removed for version 3.0.0 of the server.