-If that were not done interrupted code that's currently inspecting errno might see the wrong value. +If that was not done interrupted code that's currently inspecting errno might see the wrong value.
The existing text is perfectly good English; your change makes it less so. I'm afraid it's been too many years since high school English for me to remember the exact grammatical term for this, but "were not" is typical usage when stating a contrary-to-fact hypothetical.
(1) “IF bla bla… <comma> THEN bla bla…”
It might be more words than the bare minimum. But it helps the user separate out the proposition and the consequence.
(2) This is the dreaded curse of the passive voice (“mistakes were made”). There are many cases where an active formulation is nicer. Anyway, you can sidestep lots of the conundrums, like the alternatives here pose, by standing back and finding a different way to make the point.
> If the implementation (or you) didn’t do X, then Y bad thing could happen.
