I have an application where I need to use one pin as a digital output and analog input. I switch between input and output at a "high" frequency (>1 kHz) and it takes to much time to disable and enable the ADC-Channel all the time. When the ADC-Channel is enabled, it's not possible to use the pin as output pin and set it ti low or high. The voltage level will be at (Vgnd - Vcc)/2.
This works with the AVR Arduino boards (like the Arduino Uno) but not with the ARM version (like the Arduino due).