Trying to add money to a prepaid SIM card today. Their website is a mess. Plus and minus buttons were not functioning, so my only option was to add 15 euro. Checked the console, no errors. Tried triggering the buttons jQuery, no luck. Found a data value attached to the submit button set to 15. Changed to 10, clicked submit, and BOOM, it worked! You just got engineered!

After I paid, I was curious, went back and set it to -15, and tried it again. Unfortunately, they know about backend validation.