It was a problem that the Assembly's rules hadn't anticipated. They provide for AMs from the same party to form a group and for "unattached" AMs to form a group of their own but not for an unattached AM to join a party group.
But when Conservative leader Andrew RT Davies asked Presiding Officer Elin Jones if Mark Reckless could do just that, her answer was "yes". The rules give her the discretion to deal with unforeseen developments so she allowed a move that will affect all the opposition parties.
In a sense, the least affected will be UKIP, which remains the smallest political group. It still has five AMs, the same as the Liberal Democrats in the last Assembly. Five should still be enough to keep the chair of one of the Assembly's powerful committees.
But Mark Reckless has lost his position as a committee chair as a result of his defection. And it's likely that the change in arithmetic will cost Plaid Cymru one of its three committee chairs. That will instead go to the Conservatives, who now have the biggest opposition group.
It also gives Andrew RT Davies hope that he will once again be addressed as "leader of the opposition", as he was in the last Assembly. There's no such post in the Assembly's rules, so it's up to Elin Jones whether to bring the title back into use.
Given that accepting Mark Reckless into the Assembly group is hugely controversial in the wider Conservative party, the only party observing today's events with unalloyed pleasure is Labour. "It damages Plaid, finishes UKIP and makes the Tories the nasty party again" was one observation.