After you operate VHF contests for a while, you find that a certain group of people can be expected to be on the air in any given contest. After a few contests, you begin to remember these calls. Furthermore, if you use the W2HPF logging program, you will see these calls come up time and time again if you use the database feature. Many of these calls belong to hams we have contact with on a regular basis. And, yet, corruptions of these very calls appear in our contest logs.
Contests can be long and boring and tiring.
Or, they can be very fast and exciting when an opening happens our way. Each of these conditions produces different types of logging errors. A lot of these won't be caught until you filter through your log later, but you can take steps to see to it they don't happen to begin with. Use phonetics with your call. When you are tired, or in a hurry, or trying to pick a weak call out of the noise, it is easy to confuse one letter with another.