Well, on the topic of GM-designed monsters (are we assuming the game designers know how to do it properly?), there is no "too late" because the GM decides what the monster does.
@Professor Murder's assertion that calibration (of monster danger to PC abilities) is needed applies only at publisher-level. One GM can make adjustments, especially to her own creations. But once an ill-calibrated monster goes into print, a minor adjustment becomes a brand-wide problem.
PCs that stumble into an encounter and face death before they can run away don't have a monster calibration problem. They have a GM problem, or a we-completely-ignored-the-GM problem.