  We simulate multi-shot intensity-and-phase measurements of unstable
ultrashort-pulse trains using frequency-resolved-optical-gating (FROG) and
spectral phase interferometry for direct electric-field reconstruction
(SPIDER). Both techniques fail to reveal the pulse structure. FROG yields the
average pulse duration and suggests the instability by exhibiting disagreement
between measured and retrieved traces. SPIDER under-estimates the average pulse
duration but retrieves the correct average pulse spectral phase. An analytical
calculation confirms this behavior.
