The opposite of what it sounds like, a crumb coat is a very handy technique for keeping crumbs out of a cake's frosting.
If you've ever applied pale frosting to a dark cake, I hope you used this technique, because otherwise you pretty much have to spackle on a three-inch wall of frosting in order to make the crumbs stop showing through. If that's what you like, that's what you should do. But here's how to defeat crumbs without using more frosting than cake.
Never frost a warm cake unless you're going for a glazed, drippy effect.
Don't frost a room-temperature cake unless you seriously have no other option, and if this is the case, you're better off pouring warmish frosting over it. Cupcakes you can get away with, but generally, room-temp cake is going to tear and make a million crumbs when frosted.
Optimally, work with a refrigerated and fully chilled cake. (Super-optimal is to use a frozen one, that's an absolute dream to work with, but obviously you can only do this if there's time for the final product to defrost before it's served.)
Assuming you're working with a cake about 9 inches across, take about 1/2 cup of your frosting and thin it down with a little milk or water. Aim for a consistency that's gloppy but not runny, but there's really no getting this wrong. You want something thinner than regular frosting, but still easy to scoop up with a utensil.
Now the fun part: do the most half-assed frosting job ever! Just slap this thinned-down frosting all over all the surfaces of your cake. Crumbs will occur and move about, which is fine. Your goal is not to hide them all, but just to touch them all. You're trapping those bastards in this thin layer.
When it's covered, refrigerate the cake for a few minutes, which will solidify the crumb coat. Now you can frost it properly without fear of the crumbs getting through. You can get away with using a very minimal amount of frosting this way, if that's how you like it.
I find it very satisfying to quickly smooth on this final layer of frosting, erasing all evidence that there was ever a single crumb. Snow white frosting on a dark, dark chocolate cake is really beautiful, and with this trick, you can achieve a flawless presentation.