Shower doors and shower enclosures can leak water for many reasons. It can happen to new showers and old showers as well. Typically, shower door water leaks are caused by:
Ineffective caulking
Both framed and frameless shower doors are sealed in around the edges with caulk to prevent water from exiting the shower. Worn out or missing caulk can let moisture escape.
Broken or improperly installed shower door sweeps
Frameless shower doors generally have sweeps attached to the bottom of the door which keeps water from exiting the shower underneath of the door.
Broken or improperly installed shower door seals
Frameless shower doors usually have seals installed on the vertical edge that meets the stationary pane in order to keep water from exiting through the crack, while still allowing the door to swing.
Clogged drain holes
Framed shower doors usually have drain holes located along the bottom inside edge of the frame. This allows any water that enters the frame to drain out, into the shower. If these holes become clogged, water may drain out or overflow into the bathroom.
Unslanted shower curbs
A shower curb is the threshold which you step over to enter and exit the shower. It is usually only a couple inches in height but plays a vital role in keeping water inside the shower. Not only does it act as a barrier, the top should also have a 5-degree slant so that water that lands on top of the curb runs back into the shower.