There are two retailers around Bank, London where the Monzo card appears to crash the POS terminal when used contactless. “Patty & Bun” and “Piada”. In both cases the person behind the counter actually expected it when they realised it was the Monzo card (or “the pinkie, always the pinkie!”). In the latter case the reboot caused the card to work second time, contactless. In the former case they asked to try chip and pin and that’s OK.
At Piada it was an Ingenico IPP350 terminal owned by Barclaycard. At PAtty & Bun it was also an Ingenico, but not sure what model.