Since I got back in about 6 years ago, these two (neither are pre-war):
I have a 48/49 Leaf Musial that I owned for over two years before I started to suspect it may be a reprint
http://www.net54baseball.com/showthread.php?t=158211 I still don't know for sure, and I probably don't want to know since I paid a lot of money (for me anyway) for it. If I ever send any cards in for grading, I'll let this one go along to see what the graders think.
I also have a 40 playball Paul Waner that didn't raise any suspicions when I pulled it from the mailing envelope, but the last time I looked at it, it did look fishy. Sure enough, I compared the back to a graded copy on ebay. The text on the back was the same but didn't have the same formatting. Lines of text ended with different words.