Most linear games have a difficulty that will scale the more you play it; leading up to an encounter that will hopefully have you prepared as far as gameplay knowledge in order to complete it. Games like Half-Life 2 normally do this quite well. It's when a game crosses that very thin line of fun verse arse kicking that we have a problem I like to call the "fuck you" point. This is something that everyone must have experienced in one game or another, it's a point in the game where difficulty escalates way to quickly and you find yourself not enjoying what the designers are asking you to do in order to progress and in most cases you will never play the game ever again.
This is mostly found in RPG's where a significant amount of grinding is needed, which is not so bad; it just means you need to prepare more. Though it is a problem if you have played for going on 10 hours and this is the first real point in a game where everything is kicking your face in. The second place the "fuck you" point is found is in FPS games. The idea having difficulty based on your skill not on a number is fine, and if developers decide that throwing a small army at you as a road block which you will only struggle through is a stupid way of extending playtime.
If you're a mod developer and your reading this, keep that in mind, chances are quite a few people will never give your mod and the experience that it offers a second chance.