Switches in other bedrooms turning on different lights after I put new light up
Note:  I'm in the US, so I apologize if I get any vocabulary wrong.

I'm going to describe the picture you posted in your question because I believe you might not know exactly what each wire is doing.  I will number them left to right, 1-8 (not counting grounds/earth).

Wires 1-3:  Neutrals are not switched, so they can all be tied together.  Neutrals and hots should never be connected, and grounds should also be separate.  Wire 1 goes to the fixture, wire 2 is coming "in" from the breaker panel and wire 3 carries on the neutral to the next box (probably another light).

Wire 4:  This is the "live" wire coming in from the breaker panel.  It is energizing the screw terminal block and supplying power to wire 5 and 6.

Wire 5: Gets power from wire 4 and carries it on to the next box.

Wire 6: Gets power from 4 and carries it to the switch.

Wire 7: This is colored as a neutral, but wrapped with brown tape in the diagram to show that it is doing the job of a hot wire.  This is a live wire coming back from the switch.  It is live when the switch is on.  These two wires for the switch are known as a "switch loop" and is one of the only times (the only time?) it's acceptable for a neutral colored wire to be used as a hot.

Wire 8: Bonded to wire 7 and providing power to the lamp when the switch is on.

Now, if you confused any of the "in", "out" or "switch" wires, you could have strange results.  With the hot wires disconnected from the screws and the power on, the only hot with voltage should be the IN cable.  With the power off, you can check for continuity between the black and white wires of the SWITCH cable.  There should be continuity with the switch on, and no connection with the switch off.  The remaining cable that has no power and is not related to the switch will be the OUT cable.