Raising a mine or mason to level 6 takes a lot of money and time, and it stinks to have the deposit run out before you know it and have the whole thing collapse, so that you have to start over. My idea is that level 6 mines and masons have an auto-shutoff feature that kicks in when the deposit is down to, say, 10. It turns off the mine or mason before it can collapse, and sends you a notification prompting you to buy some refills for the deposit to prevent collapse. Players will enjoy no longer having those annoying and expensive collapses, and BB may see this as an attractive feature because it prompts people to buy refills.