Garbage in Garbage out

 You don't hear this phrase now much but when I first came up into becoming a computer programmer in college in the mid 1960s through the early 1970s this was maybe the single most important thing to know about computer programming.

Why?

Because unless your program was perfect and useful then it was garbage and counterproductive. 

Writing a good program from scratch and (not just combining subroutines that other programmers figured out) is very hard.

First you have to flow chart what you are trying to do and then analyze the steps you are trying to accomplish even before you try to write any programming in any language at all.

Otherwise, you are going to forget some of the steps and your program isn't going to work properly.

So, avoiding "Garbage in Garbage out" is still one of the most important things to know about computer programming in general.