Season 2 Ep36. Recently returned Trudy accepts Dal's invitation to spend a day at the farm, and encourages the two girls, Patsy and Cloe, to go too. Dal was also hoping for a little help. Trudy considers herself above such things and becomes angry when Dal asks Patsy. Only Cloe is willing to help out. She also notices there's something strange about Trudy. As for the goof here - it is Dal's apron strap.
Here we have an early shot from Ep 36. All looks normal. Next we have a later shot. The flap-fastener for one of the straps has come off the apron, so he's making do with just one strap. OK, so that could happen.
We next see Dal at the farm again the following day in Ep.37. And someone has obligingly sewn the fastener back on. But who? Trudy? Cloe? Patsy?

It looks like the costume department didn't sew it on firmly enough, so in the process of filming it came off. What to do? So they continued shooting that scene, then either sewed it back on for the next scene-shoot, or they did a switch with a back-up costume.

Just my humble opinion, you understand.