Summary: Lag makes it possible to collect more of a quest item than is required - and then you get stuck with useless quest items in your inventory that you can’t get rid of.
Details: I was doing a quest where I had to collect a few (I think it was 6) Operer’s Stinger (a quest item) from a mob. It was incredibly laggy, and I was killing a lot of the mob that is suppose to drop this item but wasn’t getting any. All of the sudden… I got 47 in a row. I went and handed in the quest, and the NPC took 6 Operer’s Stinger out of my inventory… leaving the remaining 41. Operer’s Stinger is a quest item, which means I can not get rid of it. They are forever stuck in my inventory.
PS: this exact same thing happened to me earlier (where I needed 9 of an item and lag allowed me to collect 21 - I forget the item name but it was a quest item dropped by Coliflower if that helps) except that NPC took all 21 so there wasn’t any problem.
