well i was doing the hullabaloo party quest and i got to the bit where i have to get hoofwritting samples so i talked to cirrums
and he gave me his hoofwritting sample but my inventory was full so i didn't get it and now i can't get it again so i can't complete the guest D: i really want to get headphones :(
I'm not sure it has to go into your inventory...
You could sell some stuff. :P
Had this happen to one of my friends.
Mods couldn't help, so they had to create another pony and do the quest again, then trade the headphones to the original pony.