What silly mistakes sometimes you do while coding or designing?

Well, no matter how much we grow as developers, designers or ourself even, we always do noobs things, It happens even the best. So, I wanna know what Junior mistakes sometimes you do while coding or designing, and please no the typical "lost semicolon" stuff. Mine, was debugging a CSS (A last minute change) for a client which site wasn't responsive; after working hours the viewport meta tag was missing.

Now you, what mistakes have you done and then realized it was something from elementary Programming/Designing school?

PD: I'm still learning English