The 16K cartridge was a standard extra piece on the ZX-81 system, along with your tape cassette recorder and your TV. People assumed if you didn't have the 16K cartridge, you must be a little kid or an initial customer still getting a good feedback from the keyboard. (Or a later customer, wanting to feel the keyboard feedback from his raw device again.) The cartridges were so standard that anyone with 1K or a third party 32K or 64K RAM pack was beset with incompatibility issues.
And Sinclair's 16K was a piece of garbage. The connection to the ZX-81 didn't have any gold plating and once the computer got blazingly hot, the contacts started developing oxide layers and getting fussy. The board was expanding, and the merest, briefest decoupling from the cartridge filled the screen with garbage. The ZX-81 did have a thin aluminum heat sink layer lining the outer black case, but its only connection to the board was a single thin aluminum prong sticking up to it.
Sinclair's reputation got cratered from its standard user experience. By the time you had typed in a thousand lines of strange BASIC out of a magazine, the RAM pack started wiggling around with every keypress. It always nailed you at your most vulnerable moment. It made everyone scream at least once.
Everyone was always swearing or lecturing: you should keep two casette tapes around, and every 100 lines, swap tapes, rewind fully, start recording, wait ten seconds, enter a SAVE command, wait a few minutes for the different-looking cassette-associated screen garbage to disappear, and then continue typing. If the permanent garbage appeared, you had to turn it off, let it cool for about fifteen minutes, rewind the correct tape, and then LOAD it once or twice or thrice until you could get the BASIC lines back off the tape.
Cheap no-name blank cartridges never worked for saving anything; you ended up starting over unless you bought (and kept buying) the sleekest, most expensive blanks. They had to take abuse well, which cassettes don't. I remember some insane procedures... always doing two or three SAVE sequences in a row, for later desperate moments when screen garbage come up the end, LOAD after LOAD after LOAD. I sometimes twirled tapes through with my fingers looking for any stretch that might have gotten crumpled or scratched, so I could dab krazy-glue on it, twirl the glue backwards into the cassette, make a new leader, and rewind to that from then on. Otherwise I quickly ran out of cassettes. My parents gave me a separate wastebasket just for them. When I did run out, I had to fish the garbage, and failing that, I would then pick out my crappiest albums and defeat their write-protects with a little scotch tape.
One trick that worked really well on the ZX-81 was the cooling system I developed. I was in seventh grade, so I fixed the problem recklessly. I filled a plastic bag with ice cubes, and left it on top of the case, at the spot where the aluminum prong "heat sink" came up to it inside. That greatly increased the temperature gradient up and down their cheap little 5 mm prong, and actually hardened the system a lot. You could type in much more code before the ice melted. (It yet crashed sometimes- this was still the eighties.) I still swapped cassettes in and out, but now I had two bags of water that I was also swapping in and out of the freezer, basically whenever that cartridge was plugged in. This system really upset my parents one day when they came into my bedroom and found a transparent plastic bag of hot melted water sitting on top of my Sinclair. I kept saying, "it keeps it from crashing!" but they never took me seriously. "Nothing keeps this thing from crashing."