The only scenario I can really think of is if you got shot and lost one of your main boost pumps and forgot to turn on the xfeed switch, so you have a major imbalance, or one of your main fuel tanks is leaking. In either case you do not want to be flying with a large imbalance as was mentioned earlier, it will cause major center of gravity problems and make it much less stable in flight. Something you don't want, especially when trying to land.