Factorio Forums

When I place pipe connecting to the thruster wrongly, it behaves in weird way. It is hard to undo those wrong action without breaking the thruster.

1. Go to the space platform. Place thruster and pipe containing thruster fuel. Put pipes to the space platform hub.
2. Link the oxidizer port and fluid system containing thruster fuel with pipe.
3. Remove every pipe except the one directly connected to the thruster port.
4. Now, remove that remaining pipe.
5. Place a pipe at that location (at the thruster's oxidizer port)
6. Then, the pipe is filled with thruster fuel. The expected behavior is the pipe remaining empty.
Removing this thruster fuel makes unnecessary hard work, such as using pump or removing and re-placing thruster. Mistake on placing pipe at the space platform make life harder because of this behavior.

Please refer to the attached video to reproduce the explained behavior.

Indeed.
The thruster has internal pipes (that facilitate connecting fuel & oxidized pipe connection points on opposite sides of the thruster).
Filling this internal pipe meant for oxidizer with fuel leads to pipeline with mixed fluids that has many quirks.

The easiest way to clean a mixed fluids pipeline may be to click on a pipe, then the Flush fluid from this pipeline button next to an unwanted fluid.
For getting access to that thruster's internal pipe to be able to clean it, connect one pipe to the connection port, exactly as is shown on your screenshot, then click the pipe.

Also see 117703 Thruster swapping Fuel and Oxidizer (mixing/fluid).