Started out as an intern at my current employer, after a few months they made me create an invoicing system...

I should have said no.

I've had a lot of bugs with it in the past, but the data-loss one has been because I send a SOAP call to our (third party) accounting system and only if I get an ERROR do I log it....

Apparently, when you put line 1 before line 0, you get a warning, but no data is processed...

Had to write a script that updated 4 months of invoice data in one go, without errors, took me a fucking week...

Lesson learnt boys and girls, never let an intern make the fucking invoicing system!