I was looking at a mongosync BF (REP-2554), and it looks like we get invalid BSON from an insert command immediately after / while a collection is dropped. The error we actually see is an InsufficientBytesError from the Go driver, which happens when it tries to read an i32 out of the BSON and there aren’t enough bytes left to do so.

The drop command for test.wildcard_cache_plans starts in the logs here: https://parsley.mongodb.com/resmoke/e8821e83ed64ef6dcb9ff38da048fa66/all?bookmarks=0,531483,531522,590010,597170&shareLine=531483. The mongosync crash is at line 531522, "too few bytes to read next component".