Quotes don't work for a certain username

[droopsnoot]

If I quote this user, the quotes don't work.

[quote=Plus4Four#]Some Welsh place names.....
Porthmadog.......Portmadog ( behave yourselves)
Aberystwyth.......Abawristwatch
Dolgellau.......Dollygaloo
In the Valleys is a town named after a noted local.......Tonyrefail, named after Tony Revail.
Kidwelly.....used to have a factory making wellies for children.
Some of these are accurate
[/quote]

If I remove the trailing # sign from their name, the quote works correctly, but seemingly not if I've already messed up the quoting.

[quote=Plus4Four]Some Welsh place names.....
Porthmadog.......Portmadog ( behave yourselves)
Aberystwyth.......Abawristwatch
Dolgellau.......Dollygaloo
In the Valleys is a town named after a noted local.......Tonyrefail, named after Tony Revail.
Kidwelly.....used to have a factory making wellies for children.
Some of these are accurate
[/quote]

I wondered if I had removed one of the brackets, but no.

[droopsnoot]

But in a new post with the # removed, it works OK

Plus4Four said:

Some Welsh place names.....
Porthmadog.......Portmadog ( behave yourselves)
Aberystwyth.......Abawristwatch
Dolgellau.......Dollygaloo
In the Valleys is a town named after a noted local.......Tonyrefail, named after Tony Revail.
Kidwelly.....used to have a factory making wellies for children.
Some of these are accurate

[Matt Dell]

It's most likely going to be some funny XML escaping logic. I'll mention it to the hamsters but they probably won't listen.

[droopsnoot]

I expect it'd be easier to add something to prevent usernames that upset it, rather than fix whatever is causing the quoting issue.