Although if you look at the actual text on the UKUSA agreement document HW 80/2 [1], rather than the short synopsis from Wikipedia, it says very clearly that the UKUSA agreement covers only sharing of "foreign communications" where footnote 3 on page explicitly excludes communications of both the UK and the USA.

1. https://discovery.nationalarchives.gov.uk/details/r/C1153691...

Optic Nerve was one of these things. Basically, GCHQ or NSA hacked Yahoo!, GCHQ used that to collect stills for every single webcam feed on the platform (this! was! back! when! Yahoo! was! still! relevant!), including a lot of intimate photos, so on. Lots of fun stuff there!

But that's a modern example. There are some pretty interesting ones. Here's a fun list of some celebrities that were the targets of a Five Eyes country and were spied on by multiple agencies as a result:

https://en.wikipedia.org/wiki/List_of_people_under_Five_Eyes...

The snowden leaks touch on it