If I remember correctly when the 128K Spectrum came out they changed some bytes in the top end of the 16K ROM that had been used frequently by games as an Interrupt Mode 2 vector table (point I register to be the high byte of the address) so that they could page in and out between the old and new ROM.

Elite was affected so Firebird (subsidiary of British Telecom) would let you get a new tape that fixed this bug but it also removed the LensLok protection code.