Eyes up!
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?

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