Alot people here still don't understand the simple system of how Steam works. So I'm trying with this post to explain what exactly happens and why it happens.

Steam servers are too busy to handle your Request
First of all you can't fix this because the Client has nothing to do with what happens. Whenever you try to Download a Game and you get this message it simply means you are not allowed to do anything, the actual meaning of this error doesnt exactly mean what it says, its more a generalized string. I'm not exactly sure how Steam checks wheater to drop this Message or not but trust me, there is no method arround it. And if you don't get this Message it doesn't mean your Patch has more quality at all, read the "Sometimes I'm able to "fully" update a Game without owning it" to understand why.

Why does the game update but only till arround 98% or less?
The answer is simple. When the Game is a Multiplayer Game like Team Fortress 2 or Counter-Strike Valve actually splits the Content into multiple parts. One part is fully shared, that means you can get it for your Dedicated Server via the HLDSUpdateTool just that it doesn't pack it into a GCF File, however it doesnt mean they dont use the same Server to get the files from. And the other part is only meant for Clients, which is usually restricted to download for free because they know you don't need those Files for the Dedicated Server. Usually it is like "game client.gcf", "game models shared.gcf", "....". The Client part is not free only the Shared one is. Thats why Steam is able to download the most of the Content except the Client part because you might not own it.

Third Party Games can be "sometimes" downloaded
Wrong, if Steam delievers for the Game a Demo they sometimes use exactly the same content, just that the Game later knows if its a Demo or not via simple API calls that Steam delievers. So if you are able to download a Third Party game it doesn't mean the Patch is any better than the other one. Whatever patch you will use it should be the same.

Sometimes I'm able to "fully" update a Game without owning it
As you can imagine they don't have only 1 Server at all, whenever you Login via Steam you get some data called "Ticket" which is for the Client and Reversing Engineers impossible to crack, because this Ticket is never decrypted on the Client. Only the Servers from Valve can actually decrypt that Data because they have the Private Key. Another factor is the actual Login and IP, if you Login to Steam it keeps 1 active TCP connection which makes all other Servers check if you are connected or not. For Example you join a Server with an Emulator you should get "No Steam logon" which simply means there is no active connection with atleast the IP or username. Now if one of the Servers is down which validates the Data they will not block the session because that would block everyone for the fact theres something wrong.

What does "Steam Validation rejected" mean?
This might be a little complex to explain but I'll try. Whenever you join an Server, the server uses the API of the Steamclient.dll which has many checks such as "Has Ticket", "Has Game", "Is Logged In", and whenever those functions return No Success, you will receive this Steam Validation rejected message. The ONLY way to bypass this, is joining cracked servers or actually host a cracked server. Without modifying the API or atleast engine.dll on the Server, you will be NEVER able to join.

"Invalid Key Size", what?!
Valve uses Data templates in the Games with checksums of the Name and Value, whenever your Game is out of date and the actual Data Table changed the checksums will not match and you will receive this message. They do not only compare the Checksum it also matters for the simple fact if the Data Table has the actual size of what it requests. Only fix is actually updating your game.

Will be there a Patch which can download all Games?
I can only say "IF" someone will be able to break the actual Private Key of the Ticket then it is actually possible, however the Data must be still valid and no one actually knows how deep the Steam Servers validate the Data. You would only know if you had the Key. And the Ticket is not just an simple encryption it is AES-256, if you ever attempted to brute-force a WinRar package you can imagine how slow that is unless you got a Botnet with the ability to Brute-Force syncrhonized.

However there are probably many other open questions, but if you ask them I might answer them if they are not too stupid. Oh and btw don't try to be stupid and explain me there are fixes we all know there is actually not one. :ROFL:.
Oh and excuse me when I got typos in the Text, I'm just a fucking nazi and no english bastard.