As well as mistranscribing the ditto mark as a surname (either "Do" or "So"), they sometimes make the opposite error. That is they misread a surname as a ditto mark, and transcribe it as whatever was above it, usually something completely unlike its true reading.
The surname I've seen this happen to is "Cox", transcribed as "Blackman". Seeing the latter name in the index, it's very difficult to identify its true value.
Regards,
Stovepipe