They use several words that were not known at the time this series took place (1192). Both "illegal" and "cancer" were used in the first series. According to Merriam-Webster the term illegal was first known to be used in 1538 and the term cancer was not known to be used until the 14th century.
The sheriff alludes to robbing Peter to pay Paul, yet this is a reference to the fact that funds from St Peters (Westminster Abbey) were used to pay for the construction of St Paul's Cathedral in London, centuries later.
Robin's bow was actually a Longbow; not the bow he uses in the show. A Longbow is, of course, long (about as long as the person using it) and is virtually straight. Also Robin never used the Saracen sword and the movement for a long sword are completely different meaning he actually wouldn't have been able to.