Mid - senior dev (L from now on) comes in on a project to help out. Starts working on creating a dashboard for the application. Work is progressing, new ideas come in, team lead (TL) is ok with everything, business analyst (BA) is also ok. The dashboard even gets thru testing (T), everything is great. In comes (A), a (probably bored) junior backend dev.

A little backstory about (A):
- seated right next to (TL)
- most discussion about every developed feature take place at (TL)-s desk, right next to (A)
- (A) was also present when discussions took place between (TL) and (BA) about dashboard
- (A) could have easily heard any number of the other team members (over 15) talk about the dashboard

Well, (A) comes into the picture ... and the dashboard (first page after login, big shiny new thing, working just fine ...) breaks. Well, breaks is a little understated. Disappears would be more exact. Cause (A) commented it out. NOT deleted from code. JUST commented out the code.

But why you ask? Because he didn't know what it did and why it was there.
No asking around, no looking up history in repository, no looking up tasks that might be related to that ... no nothing.
He's a backend dev, there's something new and unknown in the backend, the new thing has to go.

(L) didn't scream, (TL) didn't scream, (BA) didn't scream, (T) didn't scream ...
I almost screamed. This didn't happen to me, or (A) would have screamed!