ECU flashing requires a rock-solid voltage, usually between . If your battery drops to 12.2V during the writing process, the buffer verification will fail.
Try to write the immediately. Most checksum errors occur on modified files, whereas the original file's buffer should verify perfectly.
You modified the file but didn't correct the checksums in your tuning software (like WinOLS).
If KESS throws this error it starts the "Writing 0%" progress bar, you are safe. The tool has simply refused to start because it knows the file is bad. However, if it happens during the write: DO NOT turn off the ignition. DO NOT unplug the USB cable.
Are you working with a or a particular ECU model (like EDC16 or Bosch MED17) where this error keeps appearing?