As predicted, the first crahs has already happened, and nobody noticed it. May offered that EU citizens can stay in the UK and ask for citizenship if they lived there for five years already. One can certainly talk about the date from when on the count should start: May says the day when the letter for Brexit was sent, the EU wants the day when Brexit will be finally sealed.
But unacceptable from British perspective is that the Court of Justice in Luxembourgh should be the court guarding over this - to-be - right of EU foreigners in the UK. The Court of Justice is an entity of the EU and was created to especially watch over the EU treaties . That such a foreign court should also be authorizd now to watch over internal affairs of a to-be non memberstate, is a ridiculing of the British demand for claiming back national sovereignty.
London must fear that the EU uses this to create a precedence for other issues in the future when then the EU will claim this precedence to demand authority of the Luxembourgh court again.
If I were to represent the British side, I would be adamant over this so that already now the negotiations could be declared as failed, and could be ended, if the EU does not give up. That is true for the other two red lines the EU already has demanded in the past weeks as well - they all in principle are a denial of the end of British membership, and accept only the end of British rights, not its membership duties.
https://en.wikipedia.org/wiki/Court_...European_Union
That the EU already some time ago said that they see this as one of their red lines and claim the EU court should be responsible for parts of British legislation, shows that they have not understood that Brexit means Brexit: end of EU membership.
Question always is how hard a Brexit May still can sell in parliament. The bRitish side has weakened itself tremendously with those election results.