It only takes a snippet of a thread stuck between the bobbin case and the bottom of the feeder feet to create this mess. I had this happen a couple nights ago. I had been sewing all day and late into the evening- no problems. Took a break to get a bite 10 minutes after I got busy again it started nesting after cleaning -re-threading top and bottom -changing needle -putting in new spool and bobbin I shut down for the night. The next morning I went in and took out the bobbin assembly I use a gun cleaning kit to get into all those tight areas and out came a piece of thread no bigger than this ''t'' Put the bobbin case assembly back in -and I was off and running smooth as silk.