Why are UK and Welsh governments at odds over a Bill?
The Local Government (Byelaws) (Wales) Bill is a pretty simple piece of legislation, tidying up and streamlining the way council byelaws in Wales are made.
In fact, it has attracted criticism for being a relatively unadventurous first Bill to go through the assembly after it received full law making powers following the March 2011 referendum.
After a relatively smooth passage through the scrutiny process, it was passed by the assembly on 3 July with no members objecting, and was in line to be rubber stamped by The Queen and become law - the first Act of the assembly.
Assembly members applauded after the presiding officer declared the first Bill to have passed.
That applause has long since died down, and the Bill is unlikely to come before Her Majesty any time soon.
Instead, lawyers from the UK attorney general's office and the Welsh government will now be arguing their cases before the Supreme Court as to whether the assembly exceeded its authority in passing this its first law under the new powers.Principle at stake
There is no question that this is an embarrassment for the Welsh government and the assembly authorities, both of whom certified that the Bill was within the scope of the assembly's powers before it was passed by AMs. They continue to maintain that this is the case.
Sources in the UK government are stressing that this is not an "anti-devolutionist" approach being taken, but rather a desire to ensure that laws passed by the assembly are within their powers.
They accept that the point at issue is a technical one, but they are also making it clear that there is a principle at stake.
The attorney general has used what are basically "backstop" powers in the Government of Wales Act 2006 to stop the bill in its tracks.
The Act gives him four weeks from the date of the assembly passing a Bill to raise an objection and refer it to the Supreme Court.Bids for powers
It may also be a shot across the Welsh government's bows too. With a controversial Bill relating to a change in organ donation about to begin its legislative journey, there have already been questions raised about whether it relates to the devolved field of health, or raises more fundamental issues about habeas corpus, which the assembly would not be allowed to legislate for.
Under the system of devolution before May 2011, the UK government had to approve all bids for powers to legislation from the assembly.
Under the new system, the assembly can legislate right across the devolved fields without reference to the UK government.
It is only in the four-week window after AMs have voted in favour that UK ministers are allowed to intervene - the power that has been exercised on Monday.