From https://bitcointalk.org/index.php?topic=20955.msg264038#msg264038

This may be an appropriate thread to mention that the the "checksum" at the end of an address does not effectively prevent single character errors or transpositions.

For instance https://blockexplorer.com/search/1ByteCoin  shows that

1ByteCoinAddressesMatch1kpCWNXmHKW 1ByteCoinAddressesMatch1kpCxNXmHKW 
are both valid addresses even though they only differ by one character.

Similarly, the valid addresses

1ByteCoinAddressesMatchcNN781jjwLY 1ByteCoinAddressesMatchcNN718jjwLY 
only differ by one transposition.

ByteCoin
----- Original Message -----
From: Melvin Carvalho
Sent: 06/06/13 12:37 PM
To: Bitcoin Dev
Subject: [Bitcoin-development] address collision and undependability

There was a discussion on #bitcon-dev yesterday
I stated that it would be impractical to generate two bitcoin addresses, such that they differed in exactly one character (modulo different checksums).
The corollary to this is that if you find an address with a verifiable signature. Changing one character of that address would have no known private key, and hence be normally undependable.
Does that sound correct?