So, I was teaching (`demonstrating') in a practical earlier. One of the things we try to teach the students is, when doing a calculation, never to substitute actual numbers in to an equation until the last line, when you want to get a number out. Work in symbols until the very end. Otherwise you'll come unstuck. Etc. Etc. I'm sure you know all this.
Anyway, one of the students asked me why their calculation wasn't giving the correct answer. It turned out that they'd mixed up a quantity Z = 150 with another quantity whose value was 80. When I pointed this out, the student scratched out Z, and started to write in ``80''. Then realised that this was verboten, scratched out ``80'', and wondered what to write, since the question sheet didn't define any symbol for the quantity which was equal to 80. After a moment, inspiration arrived. The student wrote Z80.
There's a lesson here, but from the above it's pretty obvious that I'm not qualified to teach it.