Ever had an issue you just couldn't debug? You tried stepping through the program and the relevant parts line by line and the values in your watch window look ok, but the magic just isn't happening when you hit 'run'. You spend hours these problems, and then when you relax and take a bath or a walk the Archimedes moment happens and you realise the small mistake you made.
As for me I was trying to implement interleaved array buffer objects in Open GL today, and the meshes were rendering but the texturing just wasn't happening. After some careful deliberation and many attempts over a few hours, I gave up. When I was walking to my local restaurant I realised I didn't call glEnableVertexAttribute(). I get home, try it and it runs!
So, time wasted or a lesson well learned?
What are your most frustrating problems that were solved with something simple you forgot?