What about in cases where the commands may fail because of the actions of other users? For example, say you had a virtual market in a game and someone did a "buy from John" command while offline. they then proceed to use this purchase to beat future levels. However, once the user reconnects online, it turns out that John already sold to somebody else, and thus their purchase and everything after it is invalidated.

In your command queue pattern, what sort of ways do you handle this? Have certain "chokepoints" where the user must be online to proceed? What if you have an app where that sort of chokepoint seems to occur too frequently to make the queue useful?

Edit: I think this is similar to Fiahil's sibling comment, also relevant points made there. Thanks for the quality thoughts everyone!