just pasting the examples from the OHDSI wiki here
There is a problem that NDC can update the vocabulary using the same concepts for a different drug concepts: in the 18/12/17 release we found out that:
44865343 “Aminophylline 200 MG Oral Tablet” changed to “Cephalexin 500 MG Oral Capsule”
45092451 “Aluminum Hydroxide 200 MG / Magnesium Hydroxide 200 MG Oral Tablet [Rulox #1]” changed to “Allopurinol 100 MG Oral Tablet”
45084807 “GLIMEPIRIDE 4 MG TABLET” changed to “Naproxen sodium 550 MG Oral Tablet” (number provided is the concept_id)
so these concepts totally changed their meaning during the update