First attempt at flashing an ECU was several years ago on my Triumph Daytona 955i. Went remarkably well and the bike is still running great now several thousand miles later.
More recently I decided to have a go at flashing my 1.8T Passat with a mild performance map. All seemed to go swimmingly well; took it down the road and got it warmed up then gave it a bit of a thrashing. Excellent I thought, until on the return trip it decided to run on only 3 cylinders and the ECU light coming on all over the place. Limped the poor old girl home and in a fit of mild panic decided to go back to stock. Two attempts of flashing went wrong mid flash. Oh lord. Finally, got the stock map back on there only to find it was still running like a bag of spanners which is what I was dreading might happen. It then occurred to me that I may have suffered a coil pack failure at the worst possible time. I'm sad enough to carry a set of spares in the car as I read this was a weak spot. As luck (?) would have it I worked out which one was the faulty one by a process each coil pack in turn (yep, panic had definitely set in) and I swapped it out. Phew... Back up and running again. Probably the sensible thing to do at this point would be leave it as it but feeling bloody minded I put pave to caution and flashed the new map back on (first time this go!) and it was all good again. Touch wood, all been running fine since!