As of the the query plan cache changes introduced in 3.0 via
SERVER-15225, the database will monitor the performance of cached plans and can replan them if they are found to be less efficient than expected. Indication that this has occurred is present in the log file via the replanned:1 flag as of 3.2 via SERVER-18468. Replanning can also be triggered when the sort memory threshold is exceeded while using a cached plan.
The current replanned boolean flag recorded in the log file does not indicate which specific trigger caused the event. This can result in incorrect conclusions or additional steps to properly diagnose the situation.