I think you're confusing direct and confrontational. All five of your phrasings are combative and lack specificity (and so does the GP's questioning phrase). Simply saying or asking "this is a mistake" is not identifying the problem at all.
Maybe you implied there would be more context than just that one phrase, but in the interest of furthering the discussion, these phrasings are all better than your examples, and still very direct:
- "This approach fails under conditions A or B"
- "This code seems to assume X, where is this validated?"
- "How does this code handle condition Y?"
- "This code delivers the wrong result under condition Z"
- "This does not do what's specified in the ticket: it should do S, not P"
Maybe you implied there would be more context than just that one phrase, but in the interest of furthering the discussion, these phrasings are all better than your examples, and still very direct:
- "This approach fails under conditions A or B"
- "This code seems to assume X, where is this validated?"
- "How does this code handle condition Y?"
- "This code delivers the wrong result under condition Z"
- "This does not do what's specified in the ticket: it should do S, not P"