Aw PrimeDogs, it happens. I imagine we have all accidentally left our dogs somewhere at some point. My nan once took the neighbours' dog for a walk and forgot the dog! Another friend left her basset outside the bakery (though she did remember to take the baby home, fortunately!)
On my first agility comp I changed Merlin's usual collar to a collar with no tags (tags are a DQ). I obviously didn't do the new collar tight enough because as we walked to the ring my friend said "You do know you are not attached to your dog?" I looked down and Merlin was trotting at my side while his collar and lead were dragging behind both of us.