Emil O W Kirkegaard

It is almost true; most publishers allow you to publicly host a pre-final version of the manuscript.

do some journals disallow submission if a preprint is available on an ArXiv? If so,how do we find out which do&don't allow?

That's a different, but also important question. I've heard different stories, nothing systematic.

I've had a journal ask me to submit the paper I just put on a pre-print server. So it works both ways.

Was it a traditional (free to publish) or an OA journal? *places bet*

It was perspectives on psychological science. Turned into Lakens & Evers, 2014.

I lost the bet! But I can guess who asked you.