One thing that exasperates me is the way the general public use "maths" as a synonym for "arithmetic". Because my job does require extensive use of arithmetic, so that confusion crops us a lot. (And yes, it's a branch of maths, but I would never describe what I do as maths.)

Typical conversation with my boss:
Him: "Can you check the maths for me?"
Me: "You mean the arithmetic."
Him: "If I wanted your help on English language and grammar I'd say so."