To start with the end:
There was NO "extra" footage shown in the Netherlands when they aired I, Claudius: Zeus, by Jove.
This concerns the scene, of course, where Caligula murders his sister in a fit of madness (one fit of many, or perhaps just one very long fit). Cuts were made at the last minute behind the director's back and an extra edit after broadcast.
It was rumoured that a less edited version was shown in the Netherlands.
Now first of all I must partly take responsability for keeping the idea alive in the first place. I occasionally mentioned it on message boards such as this.
But it was reported, at the time, by director Paul Verhoeven in a newspaper article. That made it worth checking. People also reported details that they remembered seeing.
But as mentioned above, it was a false hope. An off-air finally turned up in the Netherlands last year; but it was identical. So that's the end of that story.
Only a UK off air tape might shed any light on what was edited after the premiere. (Maybe that was just the music and credits being matched better.)
The only possible takeaway is that a scene like this can have a certain effect: Because a lot is implied, viewers will add that to their memory. I don't even think anyone lied deliberately.
Secondly, it's still a horrible moment, and that probably led people to think they had seen the uncut version.
I'm still glad that I followed up - thanks to M.F. for checking.