Vishal Mega Mart — accusation

I have very worst experience with vishal mega mart vki jaipur coz before some time ago i had purchase a topper about 18 or 19 july from their but they forget to remove their tag and and i also did not even pay attention and i kept that in my bag and since then it was in my bag, but after 2 days i want again their for change a jeans which i purchased on 21 july 2017 and when i was leaving the the sound of the security beep started and security man check my bag and they they started saying that you are stealing this topper and they said either you show the bill or bill this cap and this was happened on 22-july-2017
In short they said i m a thief
If they forget to remove their tag is it my mistake
Now i will never go vishal mega mart to purchase any thing