pfSense

On 2.4.4.a.20180817.2020, make a VLAN, assign it as an interface, add the interface to a interface group. Edit the interface and disable it. From the Interfaces page, try and delete it. You will receive the following error:

The following input errors were detected:

    The interface is part of a group. Please remove it from the group to continue

Then visit the Interface group the interface was added to and see (shown in the attached screenshot) that the interface (OPT1 in this case) is no longer listed as an interface group member, despite Interfaces > Interface Groups showing OPT1, OPT2 (shown in the attached screenshot).

If you click edit on the interface group, and then only click save, then go to Interfaces, the interface can be deleted.