So I always use a case and screen protector on my iPhone. Ds uses equivalent on his google phone.
We always use independent phone shop in town.
Both of us have dropped our phones over the years and sometimes the screen has broken and we replace and screen and case.
I did this last week and got a new screen protector and case.
The screen protector wasn't the same as normal and had black edging - maybe a different brand?
After 2 days the side completely crumbled and cracked - worse than when I drop it and the screen developed spiders web cracks and then the same happened to the corner.
It had been in my handbag at work all day and was cracked when I took it out.
So I went in today and said I needed a new screen because this one had cracked unexpectedly after 2 days and as it hasn't been dropped I wondered if the screen could be faulty or I could use a different brand?
Immediately another person working there stormed over and said "you must have dropped it". I replied I hadn't or I would know why it was cracked so badly but didn't want the same protector as it seemed weak. He then accused me of having something in my bag that caused the damage. I repeated I just didn't want the same cover and did they have alternatives.
He then took his own phone from his pocket - held it aloft above his head and dropped it onto the floor. Showed me the unbroken screen protector and said again "see you must have dropped it" 
I just looked at him and said "well yours didn't break being dropped over 6ft so that proves my point that this was a weak cover because it smashed sat in my handbag on a shelf next to my desk all day"
He then said "you must have dropped it".
I just looked at him and said "whatever you may think it doesn't change the truth and as you aren't interested in me purchasing something I want I'll take my custom elsewhere - for good".
AIBU to think it's completely batshit to be so confrontational to a customer wanting to spend money in your shop and basically constantly accuse them of lying rather than actually just provide them what they want?"
I'm still a little bemused by his motives but have a new protector from elsewhere that was actually cheaper than they charge 