Add a filename containing a space and commit it. Then when using the browser to download the file from the download link, the file will be saved with a + where the space character(s) were

Example.
If the file is 'file 2.txt', you will get a link such as


and by default the browser (IE6, chrome, safari) will save it as 'file+2.txt'

I first encountered this using the windows version of fossil. However a self compiled macintosh version of fossil does the same thing.