I was getting ready for a cross-country flight and decided to download a movie to watch. After I selected the movie and made the purchase, I was presented with the error message below. 

This error state was quite the unpleasant surprise and exposed two potential points of improvement for this flow. 

(1) I was allowed to make the purchase before I received the actual product. Ideally, the user would be warned that they do not have enough storage on their device to download the movie now so that he/she can decide to back out at that point. Getting the error message after the purchasing commitment opens the possibility of paying for something that you will not be able to consume now.

(2) Assuming the business case of letting the user get this far before warning them is more important than the user unfriendliness of this design, there is one way that this error message can be improved. The user needs to be told exactly how much extra storage is necessary to continue the download. For the message above, touching OK does nothing but take you back to the screen you were on, and touching Settings takes you here:

At this point, I had no idea how large the movie file was and how much extra storage I needed to free up on my device. Knowing that I had a back-up of my mp3s on my laptop, I just started deleting songs from my phone – one album at a time – and kept checking back into the download screen to see if I had enough free storage. This was a very manual process and finally at a certain point, I was able to start the download as can be seen below.