It happens to me on Windows also, and the particular copy I was using is my sandboxed Chrome with no plugins (for sites I'm not sure of). It happened as the page was loading, I immediately got a DL requestor, and the same warning message. That pretty much had me closing the window ASAP.
I've disabled all extensions, and the same thing happens. In fact, there's also a warning that "This site is attempting to download multiple files. Do you want to allow this?"
I'm on Mountain Lion, Chrome Version 23.0.1271.101
Edited to add: This happens on page load, before there is any interaction with the page.