Declaring a complex type as memory when storage was intended (or vice versa) means updates don't persist (or unintended aliasing occurs), silently corrupting state.
User storage u = users[id]; u.balance += 1; // persists
Largest recorded losses in the same failure class — related, not the same bug:
The SaferICO scanner runs 201 detectors over your Solidity source, SAFE-0081 among them. Paste an address or the source itself — a small per-scan fee, shown before you sign, or unlimited on any plan.