Way back in university, I was trying to do an assignment for an OOP class and it had to be written in C++. I was writing a simple function since I was a beginner at it and I couldn't understand why my program wasn't working. I spent an entire practical class lesson trying to work out what the hell was wrong and in the end, I got my friend to look at it. After only 2 minutes of looking, he asked if I had declared my functions. Obviously I had not.