It would be quite horrible if you sent a large amount of Bitcoins to an address with a faulty private key! I wonder if it is possible and/or has happened in the past that bitaddress.org, the program I am using for it has such a defect.
There was a bug with v2.1's vanity address code. It was discovered fairly quickly and I'm not aware of anyone having lost any funds from it:
https://www.bitaddress.org/bitaddress.org-v2.2-SHA1-d414530eea984e9ebdd40dc27af9078cd73dc3b3.html
- critical bug fix to Vanity Wallet multiplication of a public key with a private key.
Bug was due to incorrect construction of BigInteger object. Which results in the incorrect
Bitcoin Address being displayed. Therefore, v2.1 has been taken offline.
Nobody has ever reported a paper wallet with the claim that the Bitcoin address didn't correspond to the private key.
You can check using BrainWallet:
- http://brainwallet.org <--- Click "Private Key" then paste. Can be used Offline as well.