This is a demonstration of why a code change may seem simple in theory, but when you actually do it, you can see all the side effects that you didn't consider when estimating. This is one of the many ways that estimation without looking at existing code, can be completely off...not to say it will be right-on when estimating and looking at code. I have worked a company with 8 hour planning, where every single change is by committee while looking at existing code. Mercifully, this methodology didn't last more than a few months.

The implication is that there are unit tests for those dependencies as well. Changing the signature used for them, breaks the same contracts as a wrapping function signature change. Ofc, you may already have tests to cover alternate signatures (overloading, et al) but conceptually it's the same pitfall.