Eurotunnel .
If nothing is agreed, as from 1 January 2021 national safety authorities in the UK and France would have authority over each tunnel half and EU regulations would no longer apply to the part of the tunnel that is under the jurisdiction of the UK. A legal mess.

e.g. No EU-UK recognition of professional qualifications = train drivers would have to have two sets of qualifications to drive on the British and French side of the tunnel.

No agreement on technical standards (as UK has stated it wants to diverge ) = signalling, voltage, the radio systems, the signalling system, ventilation, hydraulics. etc are no longer compatible legally.

The original Bilateral treaty of 1986 (I think) makes numerous mentions of "community law" ( ECJ). Again, massive issue.
The major issue happens when the insurer refuses insurance due to ZERO legal recourse.
GB still has its fingers in its ears singing "la la la la"

PS. I should add that the current operator of the tunnel is Getlink and is a public company so you will not hear anyone saying much about this issue as it is commercially sensitive and may upset the share price illegally in either direction.
I have no shares so am immune.

PPS. view of the EU commission and 'unofficially' the ECJ is the only way to ensure the functionality of the chunnel is a new bilateral FR-UK treaty to replace the treaty of Canterbury.

It would require an INDEPENDENT dispute resolution legal apparatus. (as ECJ is the legal body in the current treaty) and GB is allergic to "Union law".
Bear in mind, if this is not resolved, the operator will successfully sue BOTH governments and it will be for billions.

Last addition (I was unaware of). The tunnel operator Getlink DELISTED itself from the London Stock Exchange in June 2020 so, is defacto, a 100 % EU company, listed in Paris.
I'll stop adding to this thread now.
I get carried away pulling threads out of a jumper.
Fin.