When close the locker door, the little dingus in the latch slips up and over the locking pin. Remove lock in order to LIFT it over the locking pin. If you turned the locker upside down, it would likely open when locked or not. Do not try this at home.
The stitch is called a chain stitch. Pull the wrong thread, it will not unzip. Pull of the handle just pulls the loops of thread tighter to each other.
Talking with older gent some years back, vet from the EARLY days of WW 2. In the rush to get the rapidly expanded Army clothed and armed, the govt had requisitioned sewing machines from various businesses that did not make clothes, supplied them to OTHER businesses that DID make clothes, to sew up uniforms. Yeah, you guessed it.
Some of the machines were industrial machines that only sewed lockstitch. Clothes looked great- until you pulled the loose thread, and your sleeve fell off.
What we have here is... failure- to communicate.