Someone sent me an ArXiv link to a paper they thought I’d be interested in. Turns out I was reviewing the paper for Neurips.
I had to contact the Meta-reviewer and get taken off the paper.
This is a downside to ArXiv that I hadn’t even thought of: some of the the folks who are would be best able to interpret your work will likely have seen it if it’s on ArXiv. By posting before review, you may be reducing the pool of reviewers that are best able to asses the paper.