I have had multiple experiences with Clash of Clans not saving (for lack of a better word) before I close the game, and not noticing until the next time I play.
I open the game and train some troops, then close the game. Then I come back later to check on them, but they have disappeared, neither in the barracks or the army camps. I assume this is because my client did not sync with the server before I closed the game.
Note: this has also occurred while over a stable WiFi connection. My Clash of Clans is connected to Facebook and Google+.
When does Clash of Clans save and how can I make sure this happens?