Buried heads are the number one culprit. St. Augustine runners grow over a pop-up in a season or two, and once the head cannot clear the grass it waters a six-inch circle around itself while the rest of the zone goes brown. Raising it back to grade costs almost nothing.
Misaimed nozzles are the second. A head that has been kicked, mowed into, or knocked by a car sprays half its water onto concrete, where it evaporates and does nothing but grow your bill. Walk the yard while a zone is running and you will spot these in thirty seconds.
Cracked risers and lateral lines are the expensive ones if you ignore them. The tell is a patch of ground that stays soft or spongy long after the zone shut off, and grass that grows unusually fast and green in one small spot. That is your water going straight into the soil.
Dead solenoids and valve wiring faults show up as a zone that simply never runs, or one that runs and will not shut off. People often assume the controller is broken and replace it, which almost never fixes the actual problem.
The last one is not a fault at all: a controller still running a schedule somebody set two seasons ago. Houston does not need the same run times in November as it does in August. Adjusting the program four times a year saves more water than any repair on this list.
The way to find all five is simple. Run every zone one at a time and walk it while it runs. That is exactly what we do on a diagnostic visit, with you walking alongside, so you can see what is broken before you agree to pay to fix it.