Interesting tough engineering problems you ask in interviews?

Interesting thread from previous post on If candidate be asked un realistic problems in the interview How do handle 100k RPS/s scale? I believe such questions just to test if candidate can think and apply certain software engineering design principles at hand. Understand approach how do they handle corner cases ( if they can identify) technical depth, also good way to test fundamentals. For eg https://www.morling.dev/blog/one-billion-row-challenge/ https://github.com/gunnarmorling/1brc Such problems really pushes candidates to thier limits. I love to go deep in OS & database, distributed systems. Would love to see, discuss what interesting tough engineering problems you folks ask in interviews? #tech #softwareengineering