An example that comes to mind is butter. It's feminine (die Butter), but in some dialects, it's masculine (der Butter). There are maps for several other words at http://www.atlas-alltagssprache.de/runde-5/f15a-f/, but I'd take them with a grain of salt. The region I grew up in doesn't really match what these maps say about it.
Oh, I know certain people in Bavaria calling "das Butter", suspicious people no doubt! Everybody knows it is "die Butter" with an option to settle for "der Butter"!
