The situation goes like this: When I paste the bitcoin receiving address into the "Pay to" field on my (official Satoshi) bitcoin client, a different bitcoin address populates the "Label" field.
Have you changed anything since the last time you used the client where it worked successfully?
No. And it works successfully now 99.9% of the time. This has happened maybe 3 times over the last few weeks, and I've just got a new address from the customer and sent successfully (so it's not really a problem, just a mystery).
Is it possible there is a tab or other non-printable character in the clipboard contents? (e.g., try pasting to notepad or editor and then copying contents back from that paste).
Hmmm... I tried this. Nothing showed up in notepad, and the same error occurred when I repasted from notepad.
Here's the latest address it happened with if anyone else wants to donate satoshis to the mystery: