So by your definition, a fuckup is when something isn't done absolutely perfectly? Let's be realistic for a moment here; surely it would be WAY more remarkable if a company laid off 3,700 employees and exactly zero mistakes were made.

What level of error would be sufficient to warrant the haphazard descriptor? Two percent? One percent?

A company like Twitter is complex, and it's very likely that some number of people are important in a way which cannot be seen from a top-down view. Hence error correction. It's a good thing. I see it as a healthy sign that Twitter management are identifying and rectifying their mistakes so quickly.