What does this warning mean?
# acis_process_events (CIAO 4.2): The following error occurred 31 times: WARNING: pulse height is less than split threshold when performing serial CTI adjustment.
When the CTI adjustment is applied to events on the back-illuminated CCDs (ACIS-S1 and S3), sometimes one of the pulse heights in a 3x3 pixel event island can drop below the split threshold if it was above the threshold before the adjustment. In the end, pixels that are below the split threshold are ignored when the total pulse height and energy are computed. The warning may be safely ignored.