  Pulse train instabilities have often given rise to confusion in
misinterpretation in ultrafast pulse characterization measurements. Most
prominently known as the coherent artifact, a partially mode-locked laser with
non-periodic waveform may still produce an autocorrelation that has often been
misinterpreted as indication for a coherent pulse train. Some modern pulse
characterization methods easily miss the presence of a coherent artifact, too.
Here we address the particularly difficult situation of a pulse train with
chirp-only instability. This instability is shown to be virtually invisible to
autocorrelation measurements, but can be detected with FROG, SPIDER, and
dispersion scan. Our findings clearly show that great care is necessary to rule
out a chirp instability in lasers with unclear mode-locking mechanism and in
compression experiments in the single-cycle regime. Among all dynamical pulse
train instabilities analyzed so far, this instability appears to be the best
hidden incoherence and is most difficult to detect.
