It got to the point that the developers of the original Operation Flashpoint (who'd lost the rights to the name to Codemasters) put out a press release explicitly disassociating themselves and that game from Dragon Rising, essentially accusing Codemasters of using the Operation Flashpoint brand name to lure in unaware fans of the original, and even later threw a Take That at it in a promotional article for one of their own ARMA games.
Demonic Spiders: Gunship patrols. As an infantry fireteam you have absolutely no practical way to fight them (unless doing so is a mission objective and the game gives you an anti-air rocket launcher), and unlike the anemic machine gun most FPS gunships are equipped with, these guys are realistically armed with rapid-fire explosive cannons, and can shred your entire fireteam in a second or two if they spot you.
Scrappy Level: The mission to stealth-cripple the oil refinery in Dragon Rising. You're supposed to stop at an overwatch position to wait for a decoy that will draw some forces away from the refinery. This position is almost on the far side of the refinery from where you start, and you will run for ten minutes straight to get there, and if the enemy patrolling the woods sees you, it's practically game over. Once there, you have to wait two minutes before the diversion starts, then sneak into the refinery, plant a charge on the power station in the exact center, leave, and blow it without being seen. Once you enter the refinery, it's nearly impossible to avoid detection, and part of the objective is to prevent damage to the rest of the refinery, something you'll surely fail at once the shooting starts. Also, you have a time limit. Also, to emphasize your stealthy approach, your loadout consists of the SCAR-L (lower-caliber counterpart to the SCAR-H) with subsonic (in other words, low stopping power) ammunition to go with the suppressor. The easiest way to finish the mission is to blow your cover at the edge of the refinery somewhere with plenty of rocks or trees between you and the North side, where all the enemy reinforcements will come from, quickly shoot everyone further south (it's hard to do it quickly because your sub-sonic ammunition won't down single enemies until you've shot them several times) and then pick off the entire wave of reinforcements from cover as they airdrop. Oh, and your exfil point is another ten-minute run away, and god help you if some enemy stragglers get a bead on you when you're trying to get there.
Also, Mission 05 in Red River, mostly because it's WAAAYYYYY too goddamn long, you basically spend almost the entire level retreating and fighting the PLA, and it gets the worst near the end, when you have to fight several APCs and a freaking tank as well, it's pretty tough to keep the PLA from entering the FOB and destroy the enemy armor at the same time, and the last couple of checkpoints are pretty far apart.
On that note, the mission in Dragon Rising where you have to secure the airfield. Take too long and the enemy will pound the everyloving shit out of you with mortars. Once you manage to take the damn thing, you're then hit with armoured vehicles.