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That's a good point! At least it should be obvious being so much out.
I wondered how the alarm didn't go off! Mind you, despite being pretty patient and very uncomplaining I've nearly walked out of that shop a few times now due to the attitude of the staff and their shocking customer service, only stayed as they had lovely things I really wanted for my nephew.
I had it happen a few years ago with a skirt I'd bought for a wedding. I didn't wear it for a while and didn't discover the tag was on until I wore it and it was banging against my leg on the wedding day! Didn't feel like I could take it back then in case they thought I'd nicked it and couldn't find the receipt. Saying that, don't think I've worn it since the wedding!