Actually I'd like to broaden this question a bit and ask, what was the most disappointing thing you looked at a guide for that you realize, if you were just a bit more patient, you would have figured out yourself? Keeping this to ALTTP, generally...
Mine would be, in Ganon's Tower, to push the block over in the room with the Chain Chomps. Once I looked at the guide (for the first time in that play through)...I was so angry at myself. I definitely could have figured it out if I just came back later. Agh...I look forward to the day when I can 100% alttp all on my own.