This is what can happen if you don't backflush ocassionally
As indicated by the arrow, the 3-way valve is now stuck open because of a build up of coffee particles, oils, and other buildup. The valve can no longer seat properly and is leaking. Water can bypass the valve and go into the drip tray either through the 3-way or through the brewhead. In either case, the water level in the boiler is now low and will soon trip the overheat protection device. In some machines this can be reset but in others it can damage a part that needs to be replaced. This overheating can damage the boiler seals or other related parts.