NAND flash memories have bit errors that are corrected by error-correction codes (ECC). We present raw error data from multi-level-cell devices from four manufacturers, identify the root-cause mechanisms, and estimate the resulting uncorrectable bit error rates (UBER). Write, retention, and read-disturb errors all contribute. Accurately estimating the UBER requires care in characterization to include all write errors, which are highly erratic, and guardbanding for variation in raw bit error rate. NAND UBER values can be much better than 10-15, but UBER is a strong function of program/erase cycling and subsequent retention time, so UBER specifications must be coupled with maximum specifications for these quantities.