The Netting Knot
I've been making nets and teaching other people to net for a number of years. Most people can get it when you show them, but it's a little hard to get from books.
One of the big hurdles seems to be telling the difference between a good knot and the slip knot you get when the active thread slips down onto itself.
This photo shows a good knot, the netting knot, sheet bend, or weaver's knot. (I learned to weave long before I learned to net. But I never figured out how to tie a weaver's knot until after I learned to net, and realized that netting knot = weaver's knot.)
Now I'm going to turn this into the "oops" of the netting knot.
First, I push the top loop down:
Next I push the bottom loop up:
And just like that, I have what the netmaker does not want, a slip knot formed on the active part of the cord:
Why don't I want a slip knot? Unlike the netting knot in the top photo, which resists slipping, the slip knot can slide out of position and change the size of the side of the mesh:
A slip knot can be fixed by doing these steps in reverse: loosen the slip knot and push the loop back up onto the passive part of the thread (the old mesh).
When I teach netmaking, if my students don't make this mistake on their own (and many of them don't), I'll make it on purpose myself, so I can show them what it looks like, what it does on the net, and how to fix it.