I have quite a few of these so I'm doing a series.
(2 of 3) Flexi Lexi

A backend developer was tired of building data for the templates. So he created a macro/filter for our in house template lexer. This filter allowed the web designers (didn't really call them frond end devs yet back then) could just at an SQL statement in the templates.

The macro had no safe argument parsing and the designers knew basic SQL but did not know about SQL Injection and used string concatination to insert all kinds of user and request data in the queries.
Two months after this novel feature was introduced we had SQL injections all over the place when some piece of input was missing but worse the whole product was riddled with SQLi vulnerabilities.