An address showed up in my wallet that is not mine

[kiwiasian]

Is this even possible?
In my wallet I saw a strange address under "Your Bitcoin address:"
I keep all of my address organized, and I know I have never seen that particular one before.
I looked through both my sending and receiving addresses and it was not present.
More interestingly, I looked up the address in Bitcoin block explorer and it says the address does not exist in the network.

[Chick]

Is this even possible?
In my wallet I saw a strange address under "Your Bitcoin address:"
I keep all of my address organized, and I know I have never seen that particular one before.
I looked through both my sending and receiving addresses and it was not present.
More interestingly, I looked up the address in Bitcoin block explorer and it says the address does not exist in the network.

Its a new address, the client creates a new one everytime there are no more 'clean' addresses.

[MrWizard]

Your wallet generates a new address every time you receive a payment.

[silverchair]

I think it's my address, 

[kjj]

Your wallet generates a new address every time you receive a payment.

Also, almost every time you send a payment.

[natman3400]

By never existed in the network, it means it has never been used, therefore not in blockexplorer's record. Your client doesn't go around broadcasting "OHIA GAIS, THIS ISH MAH ADDRESS".

[Meni Rosenfeld]

Your wallet generates a new address every time you receive a payment.

Only when you receive payment to the address currently selected which appears in the "your bitcoin address" box.