I worry about plagiarism from the opposite angle, I've been accused before of stealing someone's idea from his screenplay - which I had read, but as far as I can tell had zero similarities to mine.
Unless you count using the same alphabet.
When I politely explained to him that my idea came from a mixture of three very famous books he got very upset and accused me of protesting my innocence too much. This guy genuinely believed I stole a sci-fi idea from him, an unproduced writer, rather than accept it was a bog-standard theme used in most long-running sci-fi shows and books.
So my worry is that just by being a member of a site like this, with hundreds of sketches published for critique (most of which I don't have time to read - sorry), sooner or later someone will turn round and accuse me of stealing their idea on the grounds the characters have the same names or something equally spurious.
I know it's a stupid, irrational fear; but it's not a pleasant experience to have to go through.
Right, now I'm off to trawl silently through your sketches for ideas.