Ability to manually add/remove from smart group to be a preference with default of No
Some unexpected results were tracked down to staff having manually added contacts to a smart group. This is with an organisation that uses Civi primarily for membership, and were seeing emails go to contacts whose membership was not current, plus other contacts being in groups that were incorrect. Background - the system was setup originally with a number of smart groups, but subsequent staff changes meant that new staff thought they had to add contacts manually. The design of contacts that are manually added & removed is that they override the smart group criteria, which is a useful feature. However, it would be better if access to this facility could be controlled by a preference, with a default of 'Not available'. At present, it can't be turned off, so this option can be quite dangerous in that it affects the integrity of the data - when users are unaware of the impact of using manual additions to smart groups. See reply & comment from ErikH here