me, testing my software:

"ok, well it works for this case.. let me make a spreadsheet of all cases and test them one by one"

*doesn't create spreadsheet

*3 use cases later..

"Ah, you know what, this seems fine, I didn't change too much...............................
...........................................................
..........................
...............
.... it should be ok"

*cough*

you know what happens from here.