For me, it's when people mention "road tax" in arguments about cycling on the roads here in the UK. It's usually middle-aged and elderly disgruntled drivers who bring it up, saying that cyclists should get off the roads because they don't pay their "road tax".
Many drivers refer to Vehicle Excise Duty as "road tax" and believe that the tax they pay on their car goes towards the upkeep of the roads. This isn't true. Vehicle Excise Duty is a tax based on your engine size, fuel consumption and emissions. This goes straight to HM Treasury and not the Department for Transport. It's not used for the upkeep of roads, and hasn't been since 1937. The roads are paid for by everyone, whether they drive, walk, cycle, or even if they don't use the roads at all. It's not exclusively drivers footing the bill and it hasn't been for 80 years.
You can start charging cyclists VED, but they don't have an engine, don't use fuel, and don't produce emissions. So you'd gather a total of £0 for your efforts. It would be a huge waste of time.
It's a really stupid argument and fundamentally wrong, yet it just won't seem to go away.
What's something which bothers you more than it should?