In the Kreisau Circle HQ, you can see a wooden board with the letters, "IDKFA". This is an old and famous cheat code for Wolfenstein's sister game: Doom which will give you all of the weapons in the game as well as all keys, full ammo and armor.
The only characters to show in all three of the Wolfenstein series (From Return to Castle Wolfenstein) are Wilhelm "Deathshead" Strasse and B.J Blazkowicz himself
If you choose to let Fergus die in the begining of the game, one of Wyatt's resistance team members will be Jimi Hendrix, although not mentioned by name, he bears exact likeness and plays guitar constantly, left hand, just like Hendrix.
In the first stage of the game (which takes place in 1946), the Allies were using B-17 bombers, even though the B-17 was officially replaced by the B-29 2 years earlier.
When Fredrich Keller was captured by B.J., he was seen in the Garage strapped to the chair he was in, but he was able to get up and stab B.J. with a screwdriver. But when the next scene comes in, Keller is again strapped to the chair.
William 'B.J' Blazkowicz: Here's how this is going to work. I'm gonna ask you one question. You're gonna answer that question in a way *I* find satisfactory and if you do not, I'll saw your head off with this here appliance, you hear me? So here it comes: Where are you people keeping the captured resistance fighters?