Corrected entry: The bagpiper in the Battle of the Somme used a plastic bagpipe chanter that had not been invented yet.
Corrected entry: Any time any of the characters try to put on a horse collar on they try to put it on upside down.
Correction: Shoulder collars are supposed to be put on upside down (widest part top) and turned at the ears - every single character harnesses the horse correctly.
Correction: Whether the chanter is actually plastic or not, it is indistinguishable in appearance to a black wood chanter, and is therefore in itself an acceptable prop. More pertinent as a mistake is that it has a modern profile. The chanter lacks a "sole" - a round disk at the end of the chanter - a part expected of a World War I era bagpipe. As a side note: While it is plausible, it is nevertheless somewhat unlikely that a West Country English regiment would be piped over the top by a Highland piper.