1. Structural deficiencies in the floor framing:
It is very common when doing a bath remodel to find floor joists that have been notched by plumbers in a manner that leaves the joist too weak to perform properly (it is my personally belief that plumbers share certain genetic material with beavers). This is particularly problematic when installing a new tile floor which will crack if the floor is not stiff enough!
2. Old corroded plumbing:
Doing a bathroom renovation is a great time to deal with old corroded cast iron or galvanized drain lines and galvanized water supply lines. Even copper water supply lines can be hidden problems if there are corroding joints.
3. Tile tub surrounds and Tile showers that aren't waterproof:
Modern plumbing codes require that showers have waterproof walls be waterproof, however, even many modern installations are not properly waterproofed (I've never had an inspector even ask about this) and certainly older installations were not typically built with any type of waterproof membrane. Since tile and grout are not waterproof in and of themselves this leads at best to problems with water vapor moving around inside your walls and at worst to outright leaks and deterioration.
4. Mold loves old bathrooms and cheap contractors:
I have never done done a full gut bathroom remodel and found mold free wall cavities. Lack of waterproofing behind tile, insufficient ventilation and other design and installation issues combined with lots of moisture on a daily basis make bathrooms a mold prone problem spot.
5. You're lucky enough to have a vent fan but it's not doing squat:
It never ceases to amaze me how many times a vent fan is installed without a duct, installed with a duct that dumps moist air into the attic or the basement, or installed with a duct to the exterior but the ducting is leaking like a sieve.
6. Someone made a board game out of your load bearing wall framing:
In a previous remodel an evil genius or a complete moron cut, hacked and generally butchered the wall studs in a load bearing wall and by some miracle your house is still standing. This is something that is much less fun to find out after the ceiling or roof caves in.
7. Your plumbing may not be vented properly:
Modern plumbing codes have changed the ways plumbers run vent lines for your sink, tub, shower or toilet. Insufficient venting can cause drains to gurgle or traps to be sucked dry, and can contribute to drains stopping up.
If your are doing a major upgrade to your bathroom consider doing a "full gut". Removing all the plaster from the walls and ceilings, and removing the sub-floor allow years of accumulating problems to be dealt with in one fell swoop. When done by a professional with expertise in bathroom remodeling you end up with a zero problems bathroom that will function flawlessly and add tremendous value to your home for many years to come.