This is something I picked up from a (brilliant) college teacher, Dr. Velez, in my abstract algebra class. One day for the third time in a row some student spotted an error in what he was writing, so he started a “typo tally” to keep count.
I do a modified form of the same thing with my students (my algebra 2 and pre-calculus — a bit fiddly to work into my lower level classes). These are the rules:
1. Every class period the typo tally resets; typos earlier in the week do not carry over.
2. If a student can spot a typo before me, the tally goes up. The typo must be mathematical in nature and not typographical, a slightly messy equals sign won’t cut it. If I spot the error and correct it myself there’s no score, even thought the students may yell “typo” while said typo is being erased.
3. If they catch three typos in one period, I bring them candy.
Last school year I got stuck 3 times, so it hasn’t been too bad on the wallet. It has the advantages that
a.) it keeps some students engaged with the simple act of writing something on the board
b.) it helps reduce situations where I find myself halfway through a problem before I realized I didn’t carry a 1 on the third step
c.) it broadcasts just how fallible I am even as an experienced teacher, and as students they shouldn’t feel bad for themselves that they make errors
(Addendum: Rules can be modified as you see fit. A teacher at my school heard about my method and kept a weekly count in his Algebra 1 class but without candy; the students still enjoyed attempting to make the counter go up as much as possible.)