So it can happen when BA have misjudged their manpower levels.
Without going into this in too much detail as this is about the CC, this manpower levels thing is a red herring. No more pilots were taken on and there was no reduction in flying - there was a change in the bidding system and now everyone works their contract, and the problem has gone away.
BA are not the devil incarnate, this working in check-in etc is a scare- mongering tactic. It takes a long time to train check in staff - cabin crew are not going to get called in to do it willy-nilly. This is about getting the crew to operate to sensible rules when there is disruption. BASSA have proved they are not trustworthy in this respect. Aircraft diverting due to snow at Heathrow should not fly home empty because the crew demand two local nights off - if it is safe for the flight crew to operate, it is safe for the CC to operate. Outside BA the CC are never more limiting than the FC, BA areis merely trying to reset to industry standard.