3rd Deadly Sin:
Threading the machine incorrectly
Even if you’ve been sewing for years, sometimes the thread is just threaded incorrectly. I’ve been guilty of this on several occasions. I can’t figure out what is causing my thread to pucker or bunch so badly. I swap needles, check my tension, and then I look at the take-up lever and realize that it’s not threaded correctly.
If you are experiencing unexplainable issues, carefully re-thread the machine and try again.