Tom went a little crazy last month downloading free games and movies in BlackBerry's giveaway, so he filled up his device storage a lot. This morning when he heard the news that BlackBerry 10.2.1 was available, he went into Settings > Software Updates and manually checked for an update and discovered one was available.
At that point in time, no error message was given that he didn't have enough storage space left to complete the download and install. Instead, over an hour later when he was 90% through downloading the OS update, it failed. Then he checked the hub to discover the notification Device Storage Almost Full - Consider moving or deleting files.
I wouldn't put the blame on Tom for this one. When he went to download the update, at that point it should be checking that there's enough storage remaining to complete the download and installation. Let's put that one on the feature wishlist for BlackBerry 10.3. [ Update: As you'll read in the comments, it looks like the OS does check, but maybe it's not properly checking the size of the entire download, so you can run into what Tom experienced. Either way, make sure you have plenty of space! ]
In the meantime, learn from Tom's experience here and CHECK THAT YOU HAVE PLENTY OF FREE MEMORY before beginning your OS update.