Yeah, code review is great...until you find out that some of your reviewers are rubber-stamping the commits from their favorites, and a large percentage of the rest are doing a sub-standard job of reviewing, and pretty much everyone is just barely finding the time to do the (decidedly un-fun) chore of reviewing code, instead of writing code. So you're back to the root cause of the problem: you have to hire good people.

Truly careless employees will (ironically) work hard to find ways around any system that you put in place to prevent carelessness. There are no magic bullets.