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.
What version?
0.5.2
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:
1EUuzvpCSE6EwLhjV48egtgjBTy4Gtsjpm