1) Not covering the countertops 

Protecting the countertops is such an important yet overlooked step when installing a backsplash. It would be such an awful feeling to work so hard to install a brand-new backsplash only to realize you chipped, broke, or made a mess of your existing counters. An easy way to prevent this is to take 45 minutes and properly protect them. I personally use a layer of painter’s plastic as a base layer, and then I use ram board on top of it. This gives me a waterproof barrier (the plastic) and another layer of protection (the ram board). If I'm working high above the countertop I’ll also have a piece of foam backer board (like kerdi board) that moves along with me as I move along.

1. Not checking the countertops for level before starting 

Surprisingly enough, sometimes countertops are not installed perfectly level. If not accounted for this can take a layout and throw it off completely. The first backsplash I ever installed had a drop of over ⅝” in 15 feet. This caused me to have top cut the bottom row of glass tile to account for this change. If you notice your countertops are out of level it is better to use a larger tile, like a 3x6” or larger to help hide the unlevel countertop. 

1. Not spending time on layout, having off-center tile or sliver cuts against finished surfaces

Layout, layout, layout. That is the name of the game in almost all cases. Spending an hour on layout will give the installation a more professional appearance. So when deciding on where to start a layout, I almost always start with trying it centered behind the sink or stovetop (because these are the most consistently viewed areas). If these cause slivers I’ll shift the layout around to get it to work. 

1. Not correcting inconsistencies in mosaic sheets

Mosaic sheets are seldom perfect out of the box, and if they are you should buy a Mega Millions ticket immediately. When working with mosaics it's imperative that you spend the extra time and correct the inconsistencies as you go. These inconsistencies could be tile set on the sheet at a wonky angle, smaller or larger grout joints, or pieces of an off-shade. Sometimes even the most trained eye will miss some on the initial set, so they should be taken out and fixed the next day.

1. Not checking to see if the tile is running level across the run, wavy runs are never desired.

A laser level will help so much with this one. But if you look down a backsplash and see that the grout joint is wavy when it's supposed to be perfectly straight, it's a sign that you weren't checking the runs as you went. I myself have done this before and had to take tiles off the wall to reset them, all because I didn't take a few moments and check.