These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The B-2 Stealth Bomber. It bypasses ground-based air defenses and can only be countered by Jet Fighters in staggering numbers. The only sure-fire way to really stop them is by razing their airfield and hope that the enemy doesn't build a new one in time. The catch? That airfield can be anywhere on the map.
Which can be solved by yet another Game Breaker, The Space Program (a Saturn V launch stand), which gives full sight of the entire map. Games can become races for this wonder and become battles to defend them against the Stealth Bomber.
Probably the least variable mileage entry, the Terra Cotta Army. Free basic infantry every few seconds. Forever.
The four "future" research slots, which become available once the tech tree is fully filled up, are this as well. Their features range from instantaneous unit production and anti-nuclear defenses to automatic conquests.
Nukes themselves are this, for fairly obvious reasons. That said, this borders on Awesome but Impractical: launching too many brings the Armageddon Timer to zero, ending the game in nuclear apocalypse.
Critical Research Failure: The Kremlin wonder looks nothing at all like the Kremlin. It does look very much like a large cathedral (St. Basil's) that is next to the Kremlin.
Also the fact that dead units linger on the map for a while. Over time, the corpses and debris gradually decay until finally vanishing from existence. Imagine the Fridge Horror that would ensure among units going through a field littered with the bones of their own countrymen.