Re: Is it on?
"Guy rings up shouting and screaming that the website is broken and it won't accept his correct data. So I suggest I bring up the same page on my screen and we go through it, field by field. We get about halfway down, to some date fields (date moved into house or something)."
There may be a second possible problem there.
Yes, the guy was clearly having a blonde moment by mistakenly trying to enter 31st September - that's the first problem, which you managed to resolve. (I've had the odd blonde moment like that of my own).
However, the other possible problem is that the company's web form didn't highlight the error for him - if it did that when he tried to submit the erroneous data, he might have spotted the problem himself without the need to call you. My clue that this may not have been done is the fact that you went through it all field by field with him - if it highlighted the field that was in error and he still didn't spot his error, there should have been no need to go through it like that.
Of course, whether my guess is reasonable may also depend on other factors, such as how long ago this was and the standards (and possibly even standard of web browser) in use.