I've just finished reading the following article on bravenewcoin.com: http://bravenewcoin.com/news/bitcoin-st ... ves-again/
This article warns users that a new strain of malware exists that replaces bitcoin addresses when they are being copied and pasted. They are changed when they are temporarily stored on the clipboard.
The malware has a list of thousands of addresses and will choose the one closest to the address you are actually trying to send money to.
So be careful, simply manually check that the address you are sending to has not changed - check some sequence in the middle, or at 1/3 of the address, too. And for large transactions, it may be a good idea to verify (ie with a quick phone call) with the person receiving the funds if the address is correct.
A new iteration of this malware will probably change addresses in emails and web pages themselves.