There are hundreds of ways to screw up a tile job, but here are a few of the most common ones. Unfortunately many contractors consider these standard and acceptable practices:
1) Mastic used on floors, in wet areas, on stone, or with large porcelain tiles. Mastic has its place: ceramic back splashes.
2) Metal or plastic mesh with a thin coat of mortar used on floors. This does absolutely nothing to stiffen the floor and often ends in popped tiles in short order. Not to be confused with a mortar bed which is much thicker.
3) Subfloor not beefed up. 5/8 or 3/4 inch plywood alone is not enough. At least one extra layer is needed. This can be plywood, Cement board, or a mortar bed.
4) Shower walls not water proofed. Happens all too often. Green board or plywood are just not good enough. Even cement board is not water proof, only water resistant. There are many ways to waterproof, Schluter Kerdi, and Den's shield with Redgard are my personal favorites.
5) Shower pans not pre-sloped. Enough said.
6) Exterior tile poorly set. Exterior installs are tricky and fail within a few years more often than not. This is due to water freezing under the tiles. Schluter Ditra, or a good pre-slope along with tile contacting mortar 100% are a must.
I'm sure there are a few people itching to add to the list so I'll leave some room:)