One time a customer came to our store and asked to repair her cellphone. The cellphone was a Nokia C8 as far as I can remember a model number. The operating system behaved in a rather strange way: It would freeze once in a while and it couldn't keep the custom settings for a more than a day. Right away, I thought the phone needed a restore. So I restored the phone to a factory default. But, strangely, this didn't fix the problem. Yeah, I've heard of all these master reset, soft reset etc. None of this helped. Moreover, the customer told me that she brought the phone to the carrier provider and their technical team couldn't fix the issue.
So I went online and found out that in addition to the above-mentioned reset methods there was another method of resetting by entering a special code which wasn't documented. I found the code and voila!, the problem was fixed. And I've learned a lot of new information while researching about that particular problem.
If I just followed the phone's manual, I would've never found a solution, so be creative and learn, don't rely only on what's written.