I made a mistake while I was playing with my MOD-WIFI-ESP8266-DEV and I messed its memory. I had to recover it by flashing firmware with AT commands.
There are plenty of tutorials about ESP8266 recovery using tools for Microsoft Windows but actually it is much easier to do it from GNU/Linux distribution. This tutorial describes what I did on Ubuntu 15.04 to flash firmware with AT command on ESP8266. Soon after recovering I flashed another firmware with MQTT support to my ESP8266.
Follow the steps below to recover ESP8266 firmware on Ubuntu or another GNU/Linux distribution (btw I guess that the process should be similar on MAC OS X):
ESP8266 Firmware update and recovery