We created a business rule with field2 (user lookup) based on field1 (type of custom entity record). The business rule (simple: if field1=value1.1, then field2=value2.1, etc.) triggering this was causing the form to always say "unsaved changes" immediately after saving.
I got sick of seeing this. No users complained, but it was bugging me.
I started by creating an "and" in the business rule condition. "Does not equal" would check if an existing value, not including null, does not equal a defined value. On the other hand, "does not contain" checks if an existing value, including null, does not equal a defined value, which is what we needed.
Might save someone some time sometime.
-BS