I go the "open a few tabs and carve multiple pumpkins at once" route so I can get it all done quicker. Did the same with the last event and the cake slice deal. Both Morostide and the last event have a glitch. Sometimes I'll attempt to carve and the page will load but I'll get an error saying I don't have the item, even though I do. I close the tab and reload and the item will still be in my invo. I'm not doing anything wrong/unallowed here, am I?

Oh, forgot to mention I got the same glitch earlier when trying to move two Apple Slices and Caramel items to my gallery.

I noticed this phenomenon in a different way, with the refresh key.

This "You don't have this item" error message happens with a stack of items that has a "Special use" (as you mentioned with the Pumpkin and Colorful Mystery Cake).

For example, if you have a stack of these item and use the refresh key to use them, you can go through multiples of them and get that error message at some point. Then it continues and repeats with another batch.

I'm not exactly sure why. It could be contiguous item IDs or something.