25% off Promo code gave 100% off to the customer. Where's the error

[Aleks2247]

I gave a promo code to the customer (25% off entire order), and at the checkout when he applied this promo code it gave him 100% off and he was only charged for a shipping. 

I checked the discount settings, I checked this particular promo code thoroughly - there's no any mistakes, everything is correct. Moreover, I asked different people from different devices to apply the same promo code - it worked normally for all of them, giving them 25% off as it supposed to be.

I really don't know how's this possible and how can this be solved? Is it possible to hack the system like this?

Regards,

Aleks