Citigroup investors learned a tough lesson on Tuesday: There's a risk in paying much of a premium for a bank still in turnaround mode.

In the run-up to Citi's fourth-quarter earnings, investors pushed its stock above $5, a nearly two-year high and a 15% premium to tangible book value. After Citi's earnings missed, and tangible book value of $4.45 a share remained practically unchanged, that premium halved.