What is happening:
I discovered that during the quest the amount of the quest item "Song Sheet Music" would change to high numbers going into the thousands and ten thousands.
The number seems to change after interacting with and giving a music sheet to every single possible pony.
The number didn't only increase but also decrease. (Possible stack overflow?)
This glitch might be causing this issue: http://phabricator.legendsofequestria.com/T2031
What should be happening:
If I remember correctly you start with 25 "Song Sheet Music" and it should decrease by one after having invited one pony.
Eventually none being left in the inventory after giving away the last "Song Sheet Music" by inviting the last pony in your journal's list.
Steps to reproduce the issue:
• Start the quest "Rehearsal Roundup" given by Pitch Perfect and Choral Codex in the Crystal Kingdom • Check the amount of the quest item after handing out an invitation to various ponies throughout the Crystal Kingdom
I don't have a screenshot after every single step, also it took some time until I learned about that glitch, so it is hard to find a possible pattern in how the number changes.
Screenshots are in chronological order (earliest first).
Amount: 25585 (change: previous * 2 - 1)
Amount: 51169 (change: previous * 2 - 1)
Amount: 36787 (change: unknown pattern)
Amount: 51161 (change: unknown pattern)
Amount: 8013 (change: unknown pattern)
Amount: 8010 (change: -3, possibly after talking to 3 ponies)
Final amount: 8009 (change: -1)
Note that in the last pictures the decrease in the number is somewhat stabilised.
Assumption: It might indicate that this glitch stopped or possibly has been fixed while I was at this quest.
But to me the quest is nonetheless broken.