National governments and multibillion dollar corporations make careless errors in their web interfaces (and now apps) all the time. This really shouldn't surprise anyone unless you know almost nothing about computers. They don't operate on magic. Considering that the algorithm is likely at least a couple thousand lines of code, there's bound to be a mistake in there. The real question is if it's a relevant mistake that will lead to an error (as it did in this case) for the end-user.
Agree. And it very much depends on the communication between the person in charge and the coder. If it's "make it so this part doesn't display until Friday", that means one thing. The coder may not know how important it is to keep that section hidden.