Leginon

DriftMonitor reports absurdly high drifts (1-10um/s) when the image is coincidentally being recorded exactly at focus and hence with low contrast. This means that it will cause an adjust target, go into drift monitor again, again reporting absurdly high drifts, and go into an endless loop target adjust / drift monitor. Drift value should be checked for sanity and measurement retried by incrementing the defocus value.

Same with autofocus
If focus is off by +2um, and fa is set to -2um contrast is sometimes very low and correlation is not possible. If this is detected fa should be temporarily set to -3um, -4um, ... and focussing retried.