A toddler has managed to travel alone for 28 miles from Powys to Shropshire.
The two-year-old boy got on with other passengers at Montgomery on Monday morning and was only noticed when the bus reached its destination at Shrewsbury.
The driver immediately contacted police, who said the child was unharmed but started a full investigation.
Insp Jonathan Tatton, of Dyfed-Powys Police, told the BBC: 'It was only when the bus reached its destination in Shrewsbury that it became apparent to the driver that the child was unaccompanied.
'The driver immediately contacted Shrewsbury police which resulted in Dyfed-Powys Police making inquires to locate the parents.'
He said two people were arrested and remain on police bail pending further inquires involving Dyfed-Powys Police's public protection unit and Powys Council's children's services team.
He said the circumstances were highly unusual, but 'fortunately the child didn't come to any harm and a thorough investigation is now under way to establish how this occurred'.