Just when you thought (2025)

If you are so experienced in life, in all its beauty and ugliness, that you have seen or heard, directly or indirectly, everything unusual that could possibly happen, so that nothing new would surprise you, then you can say the you've "seen it all". It means you have seen all that it is possible to have seen.
What you quote is not a complete sentence, but only an adverbial phrase. It tends to be used as an introduction to a sentence that would typically be omitted.

Imagine that you've just reached that stage where you think you've seen it all, and then something happens that is so weird that even in your wildest dreams you would not have thought that something like this could really happen.

is the kind of situation in which you might say this.

The "just when you thought ..." phrase (used by the author Peter Benchley in "Jaws") has caught on after being popularised by the ads for the film "Jaws" in 1975 ("Just when you thought it was safe to go back in the water ..."), and the big rise in the use of this phrase (on the "monkey hear, monkey repeat" principle) from that time is demonstrated by the Ngram .

You thought it was safe, but it wasn't, there were sharks. You thought you had seen it all, but you hadn't - there was something more (as described by Edinburgher above) to see.

It isn't a sentence. It is the start of a sentence. But titles don't have to be sentences. Here is an example using it:

Just when you thought you'd seen it all, you see something totally different. That is a big surprise, and that kind of surprise makes life interesting.

When "Just when you thought you'd seen it all" is used as a title, it often means "what I am about to tell you will be new and suprising to you".

Enquiring Mind said:

The "just when you thought ..." phrase (used by the author Peter Benchley in "Jaws") has caught on after being popularised by the ads for the film "Jaws" in 1975 ("Just when you thought it was safe to go back in the water ..."),

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Sorry to be pedantic, but that is the advertising slogan for the sequel, Jaws 2. Peter Benchley wrote the novel Jaws. I seriously doubt he was in charge of the poster for Jaws 2. It's based on his characters. He helped to write a novelization of the movie (he's listed as the third of four authors) but it's not based on a book that he wrote before the movie.