This is the exact spot we occasionally see a failure. It's happened to us perhaps 6 times in the past 3+ years. We usually see this come up when trying to add a serial to the delivery or PGI.
The problem is that MARC-CUOBJ does not equal VBAP-CUOBJ or LIPS-CUOBJ. I've seen EQUI-CUOBJ both equal and unequal to MARC.
I mention this today because it has come up once again. So far I'm unable to recreate or figure out why this is happening.