If you had a ram expansion pack fitted an overly enthusiastic key press could cause it to wobble - it was only held in place by a connector which fitted onto the zx81’s circuit board iirc, and that could cause a power cycle
I used to type programme listings† in, blind, while my family watched something on TV. When I thought I was done, I'd ask them to switch the aerial cable over so we could play whatever game I believed I'd transcribed. Having suffered from unknown RAM pack wobble during the process was gutting, but on the flipside it was amazing when I'd got it right, or almost right - and whatever mistakes I'd made forced me to learn how to debug software. A skill that pays the bills almost 40 years on :)
† from ZX Computing, or some multi-platform books from the library
In Germany later (for the C64) we had a computer magazine that listed programs in hex code. There was a special hex editor to input these programs and each line had a check sum, so we could easily see if there was a typo in our program. But at one time there was a typo in the printed hex code, oh dear...
Memotech solved this problem by having a lower-profile RAM pack that included a piece of Velcro so the connector stayed secure.
https://www.nightfallcrew.com/27/09/2013/memotech-memopack-1...
I think his dad ended up taping it in place so that it was less likely.