I have replaced washers in old systems 1000 times and this never has happened to me. This old house had a bath/shower that both stems leaked water from the stem handle while showering. I took both stems in to plumbing supplier as there was no packing in stem and no place for any. The counter man recognized that they had a new replacement that come with a oring in a place that would prevent from leaking. After finishing the job the customer says this bath/shower scalds while using it. They have the water adjusted for comfortable showering and it scalds off and on during shower. Anyone have a solution to whats wrong?
Without seeing this job ,is the house plumbed in 1/2″ copper,maybe they have a toilet leaking and refilling while they shower,you didnt say if they have to readjust the fct , or if it only last briefly.
Put the old stems back in and see what happens.Sometimes the wrong size o ring can cause a stem to turn on or off while using the fixture.If you turn the water on you might be able to see the stem turn by watching it closely.Sounds strange but it has happened to me a few times during 35 years of doing repair work.Let us know.