Submitted by: @cincuranet

There's small incosistency in casing of "integrity constraint" words.

If you try to delete index used in PK you'll get (first letters upper case):
Statement failed, SQLCODE = -607
unsuccessful metadata update
-ERASE RDB$INDICES failed
-action cancelled by trigger (1) to preserve data integrity
-Cannot delete index used by an Integrity Constraint

but when you try to deactive it, you'll get (all letters in lower case):
Statement failed, SQLCODE = -607
unsuccessful metadata update
-MODIFY RDB$INDICES failed
-action cancelled by trigger (2) to preserve data integrity
-Cannot deactivate index used by an integrity constraint