Had a similar thing once. Some how, some way, the DBA copied and pasted something wrong. Oracle DB had some odd extra syntax for left and right joins that other DBs didn't (or at least that I'd never seen). My best guess is that he auto formatted out of habit and maybe it took those symbols out.
It took a long time to find that. Because the only evidence something was wrong was that ONE of our customers wasn't being billed for ONE product. Everyone else was fine. Basically they were using it in a very atypical way. The left joins made sure to include them in the billing even because they didn't have whatever was on the right of that join. Everyone else did.