Further thoughts: plenty of competitive online interfaces do not penalise errors at all (nikoli.com and fed-sudoku come to mind). Instead, they don't let you submit any kind of answer to be recorded until you happen to get the correct one. That definitely gives you a strong advantage if you happened to have bifurcated, got to the end of the puzzle and hadn't noticed a contradiction up to that point - rather than having to spend time at the end checking to see that you haven't violated any constraints, you can get instant feedback by submitting your answer. This is part of the reason that you have 60 second checking windows during play-offs. Whilst human checkers do need a bit of time to verify correct or incorrect, it very rarely takes the full 60 seconds (indeed when I do it I tend to watch people solve and when errors are made I have usually identified them even before they submit). It's also worth mentioning that just because an error has been announced, it isn't always straightforward to identify exactly where that error is. That's one difference between something like nikoli.com, which pointed out one example of a broken constraint, and fed-sudoku which simply says that you don't match the expected solution.
To take Freddie's examples, I would also add that when you are solving competitions on paper then you will end up penalised rather strongly if you happen to miss a small bit of loop or leave a cell empty
And to take Sam's example, equally, for something online with answer codes (like the GP) if you happen to do something fat-fingered whilst typing out the code, again you end up penalised rather strongly for it. In the latter case, one reason why organisers often give very little sympathy to answer code typos is because some people do choose to spend time checking and double checking before submitting.
I tend to take the view that solving accuracy strictly matters more than solving speed, and so some degree of penalty for inaccuracy is probably appropriate in any context... but yeah, Adam is the boss here!