In fairness, I am half-remembering this from being told ages ago, so I may be getting it wrong! It may have been one particularly up-itself French restaurant that had that problem or something :)

(However, you have accidentally answered something I've been wondering about for ages, which is "is cornflour gluten-free". I thought it was, but only because I couldn't find anything saying it did have gluten in (as opposed to something definitely saying it didn't). It's nice to have someone informed make an authoritative statement on it :) )