When using a multifilesink with next-mode set to buffer, if we have an error during a fwrite call, the file stays open and if application handles the GST_FLOW_ERROR and doesn't restart multifilesink by doing a state change, the next incoming buffer will trigger and assert when trying to opening the next file:

CRITICAL **: gst_multi_file_sink_open_next_file: assertion 'multifilesink->file == NULL' failed

Closing the file after a write error allows element to recover.