I think the best example I can think of is most of the Elder Scrolls series, which will scale encounters to your level. Which, is fine most of the time. It makes for a good difficulty curve usually, where almost anything you fight might drop something you want, but won’t give it up for free.
The problem comes from if you’d leveled up on athletics, lockpicking, sneak, acrobatics, you name it. If you went to town so to speak on non-combat skills, scaling of combat encounters could get you into really bad situations.
The other example that comes to mind is the AI director. Although it’s less about making yourself stronger, and is just the game responding to you doing well, the director in Left 4 Dead, and Left 4 Deaad 2 will try to crush your advantages. You’re all super healthy, everyone has medkits in chapter two? Clearly we haven’t thrown enough tanks at you. Here, have some tanks.