The one which is very rarely of any use, but which on at least the couple of occasions I've shown it off has gotten astonishment that I find wildly disproportionate (it's easy! you just do it! ...no?) is writing back-to-front. By which I don't mean mirroring the letters, but starting with the last letter, then the next to last, and so on, until you've written a normal readable word that doesn't show the fact that you started from the end.

(I started doing this when I was taking Arabic, you see. I'd take my normal notes on most of the page, and then on the right-hand edge if I wanted to note down a new word for later I'd write the Arabic, =, and then the English equivalent -- starting from the right, of course, since that was a more surefire way to make sure it all fit than to start from the left and guess how much space I was going to need. It just made sense to do it that way!)

...Yours is a lot more useful, lol. I suspect I have one that's at least slightly more practical in ordinary life, but I can't think what it might be.