If British voters chose to stay in the EU - whenever the referendum comes - the deal upon which that decision was reached may not be the final word.
That warning came today from Martin Schulz, the President of the European Parliament which is the only democratically elected part of the EU.
On a visit to London Mr Schulz told ITV News that MEPs in the European Parliament will have to check that the much talked about emergency brake on migration - to limit access to benefits - does not discriminate between citizens of the EU.
He warned the European Parliament "is fully aware of its responsibility in this process and will step up to the plate".
In other words, he raised the spectre that MEPs have the power to disrupt or block the legislation which would kick in if - and only if - a majority of Britons voted to stay in the EU.
He used the example of someone called Claudia from Germany coming to work in the UK alongside someone called John from Britain.
As citizens of the EU - Claudia and John should get the same pay and the same benefits, he said.
And he added, if John's son Gary goes off to work in Germany, he should expect to get the same pay and benefits as any EU citizen working there.
So no matter how great the problem of migrants - and how great the pressures on public services - the principle of non-discrimination between citizens of the EU's 28 countries is, he said, more important.
It trumps the migration issue.
As Martin Schulz put it to me, "the devil is in the detail".
And it is the detail which is being poured over right now before a crunch summit in Brussels in two weeks' time.