None whatsoever. I've used yeast as much as 3 1/2 years past its expiration date with success. You have to go to a bit more effort, though. As far as I'm concerned, you should always make a starter for liquid yeast, but that's even more important for yeast past its expiration date. There's a calculator at mrmalty.com that let's you enter the date of the yeast and tells you how big a starter to make.