There is a flaw in the serial readout of the ACIS chips, causing a significant amount of charge to be randomly deposited along pixel rows as they are read out. Chip S4 (ccd_id=8) is most affected by this problem.
On account of the serial readout problem, the event file shows a variable pattern of linear streaks. These events may be flagged in or removed from the file with the destreak tool.
Technical details on the correction are available in the destreak why topic.