Print's a tricky business. Just because the dimensions are set in stone, we tend to think that everything will come out just the way we designed it. Oh, and it turns out the dimensions aren't set in stone…
The reality is that print is as device dependent as screen-based design; in fact, possibly even more so. Sure, our proportions might vary wildly but at least an RGB color is always an RGB color (well, most of the time). When you're dealing with print you have to deal with different paper stock, different inks, different printer technologies; there's even the possibility that the job will be cropped in an unexpected way. Anyone but an expert can get into a mess pretty quick.
If you're in a mess with print, or worried that you might be, this helpful infographic from UPrinting will get you on the right track.
Do you work with print regularly? What tips would you add? Let us know in the comments.