It's not common but does happen. The most common issue is when the gaining CP doesn't get informed or fluffs up the activation date. In theory the losing CP is supposed to be notified of a successful gain before disabling the account but sometimes it goes wrong.
I've had it happen to me once (had no internet for nearly 12 hours) and to a couple of friends. One of them ended up falling into the BT rabbit hole. The losing CP closed his account three days early (someone trying to get things tidied away before the weekend I suspect). The gaining CP's order hadn't gone through(blocked by the weekend). That left Saturday, Sunday and some part of Monday during which he was in limbo. At some point during that period openreach gave his cabinet connection to another customer. What knocked it into a cocked hat was that the cabinet was full. openreach refused to give him back his connection so he had no choice but to drop back to ADSL for two months.
To be fair he did get two free months free internet but that's poor compensation when you go from a 50Mb/s connection down to 4Mb/s.
Biting the hand that feeds IT
