The multiplexed serial ports
As a keen QL owner, I suggested to the company i worked for that they would make good cheap programmable serial terminals for testing our X.25 PADs.
So they got one in. Unfortunately every time we connected a QL to a PAD terminal port, the PAD crashed.
We quickly worked out that the multiplexed serial ports worked by asserting hardware flow control off each time the other port was using the hardware. Unfortunately this happened at 50Hz, and our poor PADs couldn't cope with that many interrupts a second!
Still, there's a silver lining to every cloud. When the company was taken over I was in charge of a project they canned, and so was I. However as an informal consolation they gave me the QL so now I have two :-)