I had a loss of fuel pressure once- the EDP had worked fine on the ground, but when we climbed through 1000' and switched the electric pump off, the fuel pressure did its normal drop, then kept dropping below the green arc. As it was, we turned the electric back on, turned back and landed. If we hadn't turned the booster off we would never have known- not only a problem for us, but for everyone else to take the a/c afterwards. Maybe it would never have been a problem- until the electric pump was nearly at the end of it life, and engineering said "it'll be ok for just one more flight..."