What would make a Custom 404 page suddenly stop working and show 500 instead

I have a Custom 404 page written in classic ASP that's worked well for a couple of years.
Suddenly, instead of seeing he expected nice 404, instead we're seeing a 500- internal server error.

There's nothing been changed in the IIS config (not deliberately, anyway).

Is this just IIS having a tantrum, running out of something? Might a restart fix it?
Or is this a sign of something more serious?

IIS 7.5 on Windows 2008 R2.

At Nathan C's instigation, I've remote desktopped to the server and run this locally, and I get a more detailed error response. Top tip, by the way: didn't realise you could get a different result locally!

It seems to be saying that there's something wrong with the config.

I have two custom error pages, one for 404 and one for 410.

The 410 one works fine.

The detailed error response seems to be somehow confusing the two:

Cannot add duplicate collection entry of type 'error' with combined key 
attributes 'statusCode, subStatusCode' respectively set to 410,-1.

I've looked in the web.config and the errors section contains:

<httpErrors errorMode="Custom">
            <remove statusCode="404" subStatusCode="-1" />
            <error statusCode="410" path="/custom410.asp" responseMode="ExecuteURL" />
            <error statusCode="404" path="/custom404.asp" responseMode="ExecuteURL" />
</httpErrors>

It strikes me that something is wrong with that page. It references the 404 twice but the 410 only once; there's either something extra or something missing.

But I'm not sure how it's meant to look.