In the world of signed requests, but flips are less of a concern. If the signature doesn't match, reject the call. Which implies I clearly don't mean accept literally everything. Just work in your confines and try to move the ball forward, if you can. This is especially true if you are near the user. Consider search engines with the "did you mean?" prompts. Not always correct, but a good feature when few results are found.

For system to system, things are obviously a but different. Don't just guess at what was intended. But, ideally, if you take a date in, be like the gnu date utility and try to accept many formats. But be clear in what you will return.

And, typically, have a defined behavior. That could be to crash. Doesn't have to be, though. Context of the system will be the guide.