Does your drive do any weird bit packing in its smart data? Some of mine used to do weird shit like store read errors plus total sectors read packed in one field which constantly threw errors until I added the proper data format in a config file. Without specifying a data format smartmon just assumed the drive was throwing absolutely huge numbers of errors and threw warnings every run.

Try googling the specific error message, or your drive model plus the error and see if anything pops up.