Many times I have found myself trying to figure out an issue, whether it's troubleshooting code or a problem with configuration, OS, or even hardware. After hours of banging my head against the wall confused why it doesn't work, then I realize the problem is something very simple that I should have started with. Often, people will overlook the simple solution because they are assuming it must be another cause.
Even when you have years of experience, you are not immune to this same problem. If you are a beginner, you shouldn't assume those who are more experienced than you have followed every basic troubleshooting procedure.
Always start from the ground up. Start troubleshooting with the simplest and easiest explanation first.
Code not working? Check for spelling errors, basic syntax errors, etc.
Computer not working? Reboot. Make sure everything is plugged in and turned on.
Already rebooted and you're still stumped? Reboot it again!
The rubber duck technique is based on this idea, where you explain what your code is supposed to do to someone (or something) in the smallest increments. This forces you to analyze every part of your code that you would likely have overlooked because you assumed it was working the way you wanted it to.
This most basic tip has saved me countless hours of wasted time, and similarly not following this tip has wasted more than I would like to admit.
If you have a horror story of a simple overlooked problem that plagued you, please share it in the comments.