Almost got killed because of this. Bought a bike when I was living in london (having moved from Canada). I got the mechanic to swap the brakes, so it was my “normal” right-hand-rear-brake (I knew that, if I left it the UK way, I’d inevitably flip over by instinctively jamming on the front break someday). A couple years later I needed the drive train replaced. Dozy mechanic noticed that my brakes were switched, so, without telling anyone, he switched them back to UK standard. I picked up my bike and started cycling home. A car pulled out of parking and I had to jam on the back brake (which was now the front brake), flipped right over and fell in front of an oncoming car. Lucky for me they stopped in time. Went right back to the store and had the brakes switched back (they wanted to charge me, ffs).