Derren Nesbitt was nearly blinded when the squibs in his chest blew upwards instead of outwards when filming his death scene - his character was filmed being shot in the head and the chest but in the finished film he is only shot in the head.
In the scenes where Richard Burton and Clint Eastwood climb the steep fortress walls, Eastwood moves with ease, while Burton is clearly working hard physically. This was due to the fact that Burton, who was a hard-drinker and out-of-shape by that point, chose to ride a crane (made invisible by special effects) up the wall, whereas the young, healthy Eastwood was actually climbing the wall.
After Smith and Schaffer initially climb into the Schloss Adler, Schaffer puts on a cap and Smith doesn't have one as they walk out the door of a room and into a hall. Less than a minute later, both of them are walking down another hall, and Smith now has a hat similar to Schaffer's.
When they prepare to leap into the river, the cable car appears to be over the middle of the river, perhaps even a close to the far bank. But when they do leap, the first jumper lands close the near bank and the cable car moves over the river once again.
Did the Germans really have helicopters in World War 2?
Yes, the Germans used a small number of early helicopters in late World War 2 for tasks such as transport and artillery spotting in addition to numerous experimental prototypes. Two Fa 223 Drache ('Dragon') aircraft were actually assigned to the German Army's Mountain Warfare School at Innsbruck so the prescence of a helicopter in the film is surprisingly plausible.
Who killed the 2 other members of the team?
Presumably the trio of double agents killed them because they were suspicious of them and had seen or heard something that betrayed their treason.