The bug: Some string values for an identifier property in the data objects are being sent from our frontend prefixed with a '0'. Sometimes. When it happens, it usually gets stripped away again by the time it's passed to our backend. But not always.

This 0 is never explicitly set anywhere. I even searched for a few variants of " = 0" in both the frontend and backend projects without receiving any results. You might already be suspecting where this is going.

So it turns out.

The data object which holds this value is being initialized in the aspnet (don't ask) backend and passed to the frontend, which then hydrates it. This value is always an integer number, albeit incidentally so which is why string is used as the actual type. When this object is initialized, it's hardcoded with an anonymous type where this property is set as int because I guess someone figured "it's always an int though". Being a typed language, primitive scalars can't be null objects which means the property's value becomes the concrete int 0.

Okay weird. I can think of better ways of doing this but let's just set it to string as I can't start overhauling things right now. Let's just go find where this value is somehow concatenated into the incoming parameter.

You see, this happens because at the point where the frontend sets this value, it may be an int or string depending on where it came from, and I guess someone figured that in order to cast it to string you just go prop += arg seeing as the prop is empty string and all. Because explicitly casting it or - as much as I get a rash whenever I see it - going prop = "" + arg would be too verbose and unoriginal.

Bonus round: How come the 0 only sometimes made it all the way to our backend? The thing is that this bug has been fixed before. The fix is that because this string is "always" an int, you can parse it to int before passing it to the backend in case it has leading zeroes. This path is only taken in certain views because someone forgot to copypaste their fix into all the places this is repeated.

Sometimes you find a bug and you are just somehow more grumpy after fixing it.