As Will is chasing Gisburne he jumps off a rock and seen to wear nothing beneath his robe. A few shots later there is a shot right up his robe and this time he wears underpants.
Will and Much have tan lines at the waist but are never seen to go shirtless.
When the mounted soldier who is about to be shot twice turns around, you can see one of the arrows sticking out of his chest already.
When Will Scarlet is chasing Guy of Gisburne up the river bank, Guy's black underpants are visible above the sagging hem of his trousers, and moments later the same is seen under Will's loose robes; Will is supposed to be naked underneath, having discarded his usual clothing earlier (believing them to be infected with leprosy), therefore he wouldn't have been wearing underwear, especially not modern-looking underpants which did not exist in the 13th century.
Some of the men have belt loops on their pants. Belt loops were not invented until 1922.
When Will and Much are frantically washing themselves in the water, their lips don't match the dialog coming out of their mouths.