In this context MAC stands for Message Authentication Code. An example is the bug described at section 4.27.6, although you're very unlikely to encounter that one these days. Occasionally this has been caused by server bugs.

Corruption of protocols without integrity protection (such as HTTP) will manifest in more subtle failures (such as misdisplayed text or images in a web browser) which may not be noticed. In particular, if the network is corrupting data at the TCP level, it may only be obvious with cryptographic protocols such as SSH, which explicitly check the integrity of the transferred data and complain loudly if the checks fail. It's difficult to tell from this error message whether the problem is in the client, in the server, or in between.

This probably means something has gone wrong in the encryption or decryption process. This error occurs when PuTTY decrypts an SSH packet and its checksum is not correct. Previous page next page 10.12 ‘Incorrect CRC received on packet’ or ‘Incorrect MAC received on packet’

Putty ssh for mac