Quantum computers were a good idea, until they introduced branch prediction, which, like traditional computers it didn't predict so much as run all possibilities in parallel and take the actual one.
However, unlike traditional computers, the quantum ones created alternative realities with each branch, taking the "real" one once the states coalesced.
Alternate program execution realities weren't the issue either. That is, until the darkest of those timelines learned how to inject malicious code into the others.
A bunch of technomancers in the fediverse. Keep it fairly clean please. This arcology is for all who wash up upon it's digital shore.