A couple of times now I've tried to paste and it comes out with a stripped, unclickable version of the link with ]] appended,
h t t p s : / / s m i l e . a m a z o n . c o . u k / d p / B 0 0 F 5 4 Y 6 G U
for example comes out as (and I'm pasting that without the spaces and allowing mumsnet to wreck it)
amazon.co.uk/dp/B00F54Y6GU]]
which is no use to anyone. (Also not spam, this was in reply to a request for recommendations for bluetooth headphones nearby!)
Messing with double square brackets basically doesn't help at all. Taking the 'smile' off the front and putting 'www' instead does. So many sites don't start with www now though, that seems like a bad parser to me. It should key off the h t t p : / / or h t t p s : / / and just take it straight through, surely?
Or, y'know, via a content mod to make sure it's not spammy.