I've read several combat reports from Mustang pilots, describing German Fw 190s and Me 109s wearing crosses on their wings, that were modified to resemble roundels. Most likely to confuse allied pilots, which occasionally worked.
Here's one such example of an encounter report where the pilot describes "small crosses on dirty white roundels":
http://www.spitfireperformance.com/m...-22march45.jpg
Are any photos known of such modified crosses?