Not really a hack but still worth telling:

I was working in the QA team for a big project. I tried to do some automation when I realized some radio button behaved weird... out of curiosity I checked the source and saw that there was a hidden option for a unimplemented payment option.

I was like: Let’s see how the system behaves if I just submit that form with that hidden value...

Well I was very surprised when I received the email that my order has been processed successfully.

During the investigation we found out that this bug was in prod for over two years. And it requires a one liner executed in the browsers console to skip the payment.

It was kind of a big deal and although I was (and am) still a trainee (in apprenticeship) I got invited to meet up with the client and the bosses.

It was kind of a door opener! After that they trusted me more. I have more responsibility, more interesting tasks and more client contact ever since.

To make a long story short:
Validate everything on the server side ;-)