My first job was writing a graphic editor for a digitizer connected to a computer via the COM port in DOS, using C. The display was in VGA 16 colors mode, with a resolution of 640x480. I completed it in two months, working for two hours each day after my university classes.
What did I learn? VGA 16 colors mode is cursed.
Unfortunately, I lost the source code, and so did the professor. He approached me later because he wanted some modifications to the app and asked if I could do it. I replied, ‘Sure! Do you have my source code?’ LOL.