If you access /cgi-bin/koha/cataloguing/additem.pl directly, you get:
Can't call method "fields" on an undefined value at /mnt/catalyst/koha/C4/Biblio.pm line 2133.
This is in the 'TransformMarcToKoha' function, which is called from additem.pl. It should probably do something sensible (redirect to the main cataloguing page?) if there is no record found from the supplied biblionumber.
This can happen, for example, if you change libraries when on the additem page.
This is still valid:
Can't call method "fields" on an undefined value at /home/vagrant/kohaclone/cataloguing/additem.pl line 752
Would be nice to catch this and display a human-friendlier error.