cut and paste is deciding to change a digit on its own, regardless of what was cut

Something unusual is happening with my cut and paste command, using the mouse. I ran the mouse over a section of text in one window and went to paste it into another window, when somehow one of the digits decided to change itself and it incremented by +1.. flickered, then went back to its original digit. This happened before when I cut and pasted data from a mathematical program, but sent the wrong answer (it was off by 1 digit) to my mathematical friend, who later asked me about my answer, since it was not correct.

Could a bug have gotten into this cut-and-paste operation?