I recently hooked up a small buzzer to my ESP8266, connected to Pin D9 (GPIO3) and GND. Works great with either the tone() function or digitalWrite().
When trying to re-compile and re-upload my sketch when the buzzer was connected, the upload could not connect: espcomm_sync failed
Turns out, the D9 is also the RX0 line. This confuses the ESP.
Lesson learned: Use another pin
Question left: Is there another solution? Adding a pull-up resistor?