When I was involved in the teaching of a course that allowed this, one student basically reduced all the course notes onto a single two-sided page, except they also had flaps on it to fit more in. The exam invigilator took it from them deeming it (correctly imo) to not be in the spirit of the allowance. We decided we'd mark their exam, and then determine what we should do about this cheating-cheat-sheet. Given they got something like 5% in the exam anyway, we decided to let it slide.

My own experience of making up those cheat sheets is the same as yours, the effort put in to create them is an excellent teaching tool in itself, as you're studying and distilling all the stuff you don't know well, and by the time you're done, you hardly have to refer to it in the exam anyway.

For physics, they'd just give us a page with all the formulas but with no context aside from the letters in them, so if you knew roughly what it should look like, you could find it and you're not at so much risk of making a basic error under pressure.

I still have a couple of the cheat sheets my study group compiled -- they're so fun to look back on. One for Computer Organization is almost unintelligible to me now; we were so proud of the density of that sucker.

did your instructor also allow a desktop microscope?

One of the more creative tricks I remember was writing a cheatsheet in red and blue text overlaid on each other and using 3D glasses to read it, which allowed you to squeeze more on. Never actually did it, so I don't know if it was useful...

This was common in my engineering school too. I enjoyed creating the cheat-sheets more than any other form of studying, because what better way of learning three chapters of thermodynamics than having to distill them into 10 lines of text on a notecard.

I always permitted/encouraged this when I taught (I'm retired now). Every once in a while, we'd find some enterprising student selling pre-made cheat sheets, which were of course of little value to the purchaser.

I can imagine that! In my university, we insisted on having the sheets be handwritten, which (at least) acted as a dampner.

At one point in college, I printed "Don't Panic" in large letters on one of these one page cheat sheets.

This raised eyebrows of the TA's proctoring the exam, but like, if you don't know the stuff, a page isn't going to help you at that point.

I recall at some point that I started writing that type of cheat sheet, even if I didn't get to refer to it in the exam, for that reason.