It happened to us too with Unidentified Allagan Tomestone. 1 guy was suppose to need it and everyone else pass. The guy who was needing saw the other loot drop the 2 allagen pieces and then left the zone then the last person passed and it was lost.

This was player error he should of not left the zone until it dropped. Lesson learned and it didn't happen the next week. SE has a strict policy of not giving you items that were never in your inventory. Guess got to be more careful with lotting passing and leaving the zone.