As many users complained about, sometimes after a long time of transcoding was done, an error occurred because of a muxer error and this wasted their time. 😀
So in 0.6.0.3940, an automatic testing phase was added before the actual transcoding starts. The testing phase will convert 1 second (can be altered at Settings->Overall->Task->Testing phase duration) of original content to see if the result file can be successfully created. Only on success, the whole file will be processed.
By default, the testing phase will be activated only when external muxer is to be used.