One of the big problems is Timing. Yes, it does make a difference WHEN you use the LDS.org site. Even though LDS.org is the largest Christian website in the world, if many people all decide to work on their lesson for next week or research their family history on a Sunday afternoon or evening, the site will become overwhelmed and start sending out error messages. One of the system's common error messages when it can no longer handle the traffic on the site is to declare that it is "Down for Maintenance". Of course the site is not down for maintenance on the highest demand day, Sunday. Error messages can also occur at other high traffic times such as General Conference.
Here is a message which can shake you up and can occur when all you've done is enter the LDS.org website. Of course you aren't doing anything that is "Forbidden". The computer is just overloaded.
You don't have permission to access / on this server. Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.
I find that FamilySearch.org works a lot better with less error messages and faster downloading and searching when I use it at a less busy time such as weekday mornings or afternoons. It also helps to plan and study for next Sunday's lessons a week ahead. Lets face it, LOTS of people are trying to gather their lesson materials on Sunday morning!
But timing does not account for all the 404 standard HTTP error messages, or all the Page Not Found error messages. The question still remains, "Why do some people get loads of them and others almost never see an error message?" The answer has to do with their computer. If a person has an old operating system that can't handle all the demands of a complex, high tech site such as LDS.org, then there will appear many error messages. Also, if a person has a full cache and is heavily loaded with cookies, they will receive lots of error messages. Their bogged down system simply can't handle complex requests such as playing videos the on LDS.org. Clearing your cache and cookies is the most common recommended procedure to help improve computer performance. Try it. It really works!
So if you are getting swamped with error messages, try accessing the site at a different time, try getting out of LDS.org and then reenter, and if that doesn't work, try a little housekeeping - clear you cache and cookies. Using a little common sense about timing and having regular computer maintenance including keeping your system updated, will eliminate most of your error message and help you have a good experience using LDS.org.