I've never had a problem but my mom has- especially ordering solids. I do have a more expensive high def laptop screen and hers is just a normal HP monitor, so maybe that makes a difference. We have put them next to each other and colors on her screen look darker than on mine.
I also agree with checking the ruler thing- I just ordered a bunch from Fabric.com and made sure to check them all. I like the huge scale fabrics myself.