The two transfers that froze
A banking app moves money between accounts, locking each account while it touches it. Thread A is moving money from account 1 to account 2, so it locks account 1 and then reaches for account 2. At the same instant, Thread B is moving money from account 2 to account 1, so it locks account 2 and then reaches for account 1. Now A holds 1 and waits for 2; B holds 2 and waits for 1. Neither will ever let go. The app just hangs — no crash, no error, frozen forever.
You haven't read about this exact system. That's the point — walk the three questions a designer asks: what's really wrong, what fixes it, and what does the fix cost?
Diagnose
Read past the story. What's the REAL problem — not what the feature does, but what's actually going wrong?
Look past the story. What's the REAL problem here?