I was play testing and got kicked from single player with this message

looking in NBTExplorer shows that the backpack is storing air inside air if there isn't anything else in the slot

that is only a few layers

versions are:
minecraft 1.19.2
pack it up 0.3.0+1.19

the log because I forgot it
I did try to log back in once
replaced with redacted copy
https://gist.github.com/800020h/ea3d9909eb8a5b91af65c283c873590b

it may have been caused by supplementaries 1.19.2-2.2.46

I found out there was an issue that could have caused it
this is the commit that fixed it afaik
MehVahdJukaar/Supplementaries@dbca5a2

the quick test I performed just now didn't show signs of it happening without supplementaries