In rare circumstances, your keyboard/device can become unwritable, and dfu-programmer will give you an error like this:

Erasing flash...  Success
Checking memory from 0x0 to 0x6FFF...  Empty.
Checking memory from 0x0 to 0x607F...  Empty.
0%                            100%  Programming 0x6080 bytes...
[ X  ERROR
Memory write error, use debug for more info.

Currently the only way to solve this is to reprogram the chip via ISP. This requires another device to be hooked up to a couple of exposed pins on the PCB. There is a guide on how to do this here, and this is where things are on the Planck PCB.

An example command to flash the board once things are hooked up is:

avrdude -c usbtiny -p m32u4 -U flash:w:planck_default_rev4.hex

Research is still being done on why this happens, but here are some cases:

• make -f Makefile.rn42 dfu and not the dfu-programmer commands worked for @tybenz - also see the hhkb keyboard on tmk
• Doing a force erase works here
• dfu-programmer atmega32u4 erase --force works here as well [DE]
• Unresolved, but some data dumps