"Hey, what are you changing in the code?"

"Nothing that will affect your side"

Nice! Did anyone not see that coming?

Had some guys that supply us a library internally (let us call it BadLib) say the same, then our code stopped running.
We (GoodLib) depend upon a fixed ABI with BadLib, so that clients can drop in an updated GoodLib with an old BadLib, or vice versa, without forcing us to recompile for every minor release.
This has worked well since way before I ever joined the company. Then they started letting another team get access to BadLib code, to manipulate it to their preferences, which they did thinking it didn't affect anybody else. Well, let's just say it's three months down the line, and we have problems almost daily with interface changes we were not aware of...