- the omission made the question trivial, and
- you didn't see it within a few minutes
it is probably better to ask a different question. If there were only one answer, an option would also be to ask for the opinion of the answerer whether they mind updating their answer. When looking for examples of some phenomenon such an update may not tax the answerer unduly, and the overall quality of the material on the site is enhanced by having a better question and an answer addressing several cases.
If the omission made the question trivial, the answerers should have IMO checked before posting. If an answer was posted too soon, particularly within the five minute grace window the system gives to all the askers, then they have IMO no basis to their complaints (opinions may vary here).