Sounds like "Don't count chickens before they hatch" or, in this case, don't allocate time or money or sign deals or promise things before are darned sure can do them -- better yet, have those things already done.

Or basically have a plan that has the startup go live and get revenue enough for break even before making promises. Then, when one of the unexpected things happens, the launch date is just pushed out another month or two but don't get into trouble with investors, bankers, lawyers, employees, etc.