What if the Germans waited to deploy their magnetic mines until the invasion of France? Then, when using mines to block ports during the invasion of the Lowlands and France, they drop several hundred/1k+ at Dunkirk, which the British ITTL will have never encountered and have no countermeasures for?
OTL there were only about 2 days that the Luftwaffe could fly, but that is plenty of time to spam the port with mines using multiple sortees. The British OTL were seriously afraid of the magnetic mines when they first encountered them because they had no counter measures and didn't really have any effective method for sweeping them until IOTL they captured one that was accidentally dropped on land near London in 1939.
Without the critical methods of sweeping learned by this capture the British would suffer unacceptable numbers of losses in the evacuation, which means it stretches out for several weeks or is abandoned altogether. What does this mean politically and militarily? Do some men still get out, does the RN still risk the evacuation?