The ZX Spectrum sold with either 16 or 48K RAM, necessitating an optional 32K memory bank which was achieved in a characteristically (for Sinclair) cleverly frugal way: with half-bad 64kbit DRAM chips, i.e. that had a defect on one half of the chip, leaving the other half usable, so the chip could be sold at a discount.
This makes so much sense that I'm surprised it wasn't a recurring pattern in later years, but it doesn't seem to have been. Tandy did the same thing with the CoCo, but that was around the same time.
In fact, it may be possible to pinpoint the timing more exactly. Acorn designed the Electron as a follow-up to the BBC Micro, just late enough that it wanted to use 64kbit RAM chips instead of 16k. And certainly it would have been great to have 64K in the machine. But their architecture was designed for 32K, existing software wasn't designed to take advantage of extra, they would've had to design a bank switching scheme, and above all else, the Electron was supposed to be cheap to compete with the Spectrum, so they ended up using just four chips to provide 32K, which meant every memory access took two cycles, which made the machine much slower than it would've been.
So this was an ideal case for half-bad chips. Why didn't Acorn use them? Maybe they just didn't think of it in time. But the Spectrum was designed starting in 1981 for release in 82. The Electron was designed starting in 82 for release in 83. (And was a financial disaster because the Ferranti ULAs still had trouble working reliably at 16 MHz, so they didn't get volume production until 84, by which time the market peak was past, so they ended up with warehouses full of unsold machines, but that's another matter.)
Did half-bad RAM chips disappear between 81 and 82? Was yield really that close to perfect already? And why didn't they recur in the 256kbit and subsequent generations? Or was something else going on that I'm not taking into account?