The ZTE F660 has a bootloader and a primary firmware partition. If you flash the wrong file (one intended for a different hardware version, e.g., V1 vs. V5), or if the power fails during the update, the device will "brick." It will become a paperweight. Recovering a bricked F660 often requires a serial console (TTL) cable and soldering skills that are beyond the average user.

: Ensure the firmware version matches your specific hardware revision (e.g., V7 vs. V8 or V9) to avoid hardware mismatches.

Ensure your router is connected to a reliable power source. A power failure during an upgrade can "brick" the device.