How do bugs come about in programs

Answer 1

They come to poorly made programs without instructions/limitations.

Answer 2

They are often found in simply made programs that haven't been implemented with great Algorithms

Answer 3

Bugs are most often introduced as and when the code is initially created, because it is very difficult for a human to think of all possible combination of data and events that may be handled by the program.
This is why it is so important to test a lot and systematically after each code modification, to find and fix all bugs.