Boot loop after flashing android 6.0 marshmallow on nexus 5 follow up
I too faced this issue and issuing the fastboot commands manually (instead of relying on flash-all) and it worked well.

These are the commands to use (just forget everything and make sure these commands are executed in this order with the appropriate .img file available):

fastboot flash recovery recovery.img
fastboot flash boot boot.img
fastboot flash cache cache.img
fastboot flash userdata userdata.img
fastboot flash system system.img
fastboot reboot