You've seen this many times. A cool drink on a hot day can form condensation on the container. That's one of the reason's we have coasters. To prevent water from dripping onto the table.
Now, step into the attic. Trace the large black line from the outside over to your inside unit. Is the insulation missing anywhere along the cold pipe?
If so, the cold pipe will sweat and drip water onto the ceiling below...and just like the cold drink, the condensation will collect in an area where you don't want...YOUR CEILING!
And...you could be fooled into thinking you have a roof leak.
FIX IT :: Simply add missing insulation to this cold pipe. The insulation will prevent condensation and therefore dripping water onto your ceiling or wall.
This inspection is easy to perform. But...should you need help, call a professional home inspector. They're trained to recognize these and a host of other conditions in your home.