The most useless thing I worked on was as a working student. The whole org was developing on one specific product (around 200 people) for around 4 years. Our company did a strategic acquisition and the acquired company made the same kind of product our org was doing. Their product was already in use, so they disassembled our org overnight and moved the affected teams to different projects.

The software architect of my team was heartbroken because of this. He was fully committed and it was exactly his dream to work on such a product. The last 4 years he was working his ass off and everything went to waste. I still feel bad for him.