The permissions for the directory are not restored after the FTDC thread throws an exception. The cleanup from the unittest::TempDir::~TempDir() destructor logs a permission denied error but continues.

// Remove RW permissions from the directory to force the FTDC thread to throw.
boost::filesystem::permissions(dir(), boost::filesystem::no_perms);

https://github.com/mongodb/mongo/blob/af78ce10f41a0867decb8e93bfc3f23fcdd585d2/src/mongo/db/ftdc/controller_test.cpp#L494-L495