A software bug is an error, flaw, mistake, failure, or fault in a computer program or system that produces an incorrect or unexpected result, or causes it to behave in unintended ways. Software bug - Wikipedia, the free encyclopedia

A glitch is a short-lived fault in a system. It is often used to describe a transient fault that corrects itself, and is therefore difficult to troubleshoot. The term is particularly common in the computing and electronics industries, and in circuit bending, as well as among players of video games, although it is applied to all types of systems including human organizations and nature.
Glitch - Wikipedia, the free encyclopedia
Yes I know the terms quite well, but they could be used ironiclaly... "Do you have glitches in your system, buddy?"

Originally it was a colloquial shortening of "hallucination" and gain a wide spread as a description of phenomenon when "you see something wrong or unexpected".
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Thanks!