It is a Test data selection technique. The selection
criterion is to pick values that seem likely to cause errors
Error Guessing is based mostly upon experience, with some
assistance from other techniques such as boundary value
analysis. Based on experience, the test designer guesses the
types of errors that could occur in a particular type of
software and designs test cases to uncover them.

Error guessing is basically guessing the error.Looking at
the application we might be able to guess the error. It can
be mostly like cosmetic defects.This can be easily done with
experience.

It is a technique which is used by the people who have prior
knowledge on the application

Against to the requirement specification in the application 
build

for e.g, user name 4 to 16 characters only

now we will test the min-1, max+1 values