Programmer only uploading with its own power source Can anyone explain to me why the following occurs (See image below)?

When I connect the pins according to situation 1, everything works fine and I am able to upload my project. However when I connect the pins according to situation 2 it gives me the following error:


  wrong direction/command: 0x00 0x08, expected 0x01 0x08 esp8266




Both supply the same constant 3.3V and after the project is uploaded it runs with the pins connected as in both scenarios.

(All the other factors are constant in both scenarios. Think of pulling down IO0 etc.)
There's no problem with supplying a separate power supply to the ESP8266 but you always need to connect the grounds between the "programmer" and the ESP8266.