Some players might tend to use up the bonds whenever they have something unlocked, actually this is unnecessary, because not all of them are of immediate use. Sometimes this could kill you off (you might not lose the station, but could cause some serious troubles). It is a common mistake to spend all the bonds and suddenly a commuter with cardiac arrest pops up (oops). A good practice is to check the to-do list on the top right hand corner, if it ask you to build a rat prod, your station is probably gonna be infested with rats very frequently, that way you can prepare for it.