Your 5 and 6 combo point finishing moves won’t consume a stack of Deathstalker’s Mark on your target for extra plague damage if you have the Darkest Night buff.
This most often occurs whenever you leave combat and gain the Darkest Night buff and enter your next combat with an Ambush from stealth as your opener. The Ambush applies a mark to your target but you cannot begin consuming the stacks of the mark until AFTER you reapply the mark with a 7 CP Envenom, the Darkest Night buff fades, or you right-click cancel the Darkest Night buff early.