Saw this on Insiders by 2 dif elites:

"I recently used the mobile app to check into a hotel. Several hours later, my schedule changed and I shortened my stay by cancelling the first night. I did that by calling the 800 number and cancelling night #1 well before 6pm on the day of arrival (my corporate agreement with Marriott remains 6pm day of arrival). When I checked out of hotel several days later, you guessed it, they had charged me for the first night as a no show. When I had explained that I had cancelled the room, and had a confirmation number, the attendant at the Tysons Corner Marriott located in Mclean, VA stated "but you had already checked in using the mobile app"... no kidding, I know what I did and didn't need to be reminded. I basically said I dont have time to argue this, just adjust my bill so I can check out to which he responded "you've already been charged and I need to give this to accounting and that will take a month".

"I was charged for a no show by the Residence Inn Ballston Arlington by DCA even though I had called Marriott (using the number in the App!) who rebooked me to another Residence Inn!"

So another potential downside to the app.

Cheers.