CoD1, in the early missions in St. Mere-Eglise or whatever, when the rest of the paratroopers land and you have to run across a field raked by artillery fire. Nothing says "pwnt" like running straight into a shell explosion. Then later, when you've taken the town and the tank shows up, where you need to run about a hundred feet (estimated) to a Panzerfaust, little or no cover to speak of, German soldiers coming out the wazoo and the bloody tank machine-gunning the road. Took me a good few tries before I dodged enough bullets to make it to the church, then assumed the game was realistic enough for a Panzerfaust to act the way it's supposed to so I aimed above the tank to compensate for range. Of course, it works like a rocket launcher instead of a ballistic grenade - that's what I call a "dick move" in the face of people who actually know shit about WWII-era weapons beyond the Thompson and MP40.
And then there was that bloody Russian level. The first one, that Wikipedia says is similar to Enemy at the Gates, where all you get is one clip and no rifle, and you can't pick up dead guys' rifles. While I do agree that that has a sense of realism, since you're supposed to be some peasant fresh off the fields of Outer Bass-Ackwardistan and not Tommy Testosterone Tits shooting all the machinegunners on the ridge with sniper-like aim, but the intensity of the artillery barrage just sucked Bawls(tm). It took me at least six tries just to get up to the wall with the sniper behind it, and even then I had only a glimmer of health left over.
Second Soviet mission sucks as well, I tend to hang back and pick up every rifle in sight, but apparently the commissars think that trying to fight worth a damn is high treason and shoot me in the back, or I just get hit by stray fire from a friendly machinegunner, I could never tell.
Or in Homeworld 2, the mission where you have to fight the Movers while the Mothership is out of control - those things are bloody impossible to kill. In the end I went back and started the mission with a full complement of Interceptors, Flak and Torpedo Frigates, and the bleeders STILL took out roughly 80% of my forces within the first two minutes.