Had a lab for class that asked for something super simple in Java. All you had to do was submit your source code when it was done, go through some test conditions, and if it passed you were free to go. I finished the work in 10 minutes, yet spent the next 40 trying in vain to submit it. The work was fine, I knew it was right, but no matter what I changed, the damn thing wouldn't accept my work. After countless attempts and changes, getting the professor herself over there looking at it, I finally figured it out.

I was submitting the class file by mistake.

Some errors are harder than others, I guess.