Well usually you don't make code of that quality, of course. But when you have one or more of the following:

you have a tight deadline,

you are editing someone else's code,

you are adding functionality to older code,

you are coding tired,

you are coding without a prior design,

chances are that you make mistakes. Usually, you rely on errors and warnings. Yet, if those are not available, it can be hard to solve an error presented.