Where does this wire go?

[Exuptoy]

I've had a few late night knocks on the front door. My dad / wife / husband (insert anyone you like in here) bought new light today and put all the reds together and all the blacks together and now nothing works / fuse blows / hallway light comes on.
One of the lads from work got me to call in on the way home as when he turned on the light near the front door the kitchen lights at the rear of the house came on!

 

[WorkshopChris]

The other one that used to happen is when someone decided that they would modernize there own consumer unit and swap a neutral over on a dual rcd board and then find the rcd would not hold in.

 

[Robotstar5]

The other one that used to happen is when someone decided that they would modernize there own consumer unit and swap a neutral over on a dual rcd board and then find the rcd would not hold in.

The popular fault with DIY CU upgrades is a "borrowed neutral" for the stairs light, RCD won't stay in

 

[Exuptoy]

Funny you should say that, my uncle used to be renowned around these parts for doing cheap rewires (he isn't a sparky either!) and he knocked my door one Sunday morning and his exact words were "I've just finished this rewire and the RCD keeps tripping". I said you've installed a splitway board with an RCD one side a switch fuse on the other haven't you? He said how do you know. Yep you guessed it he'd put some neutrals in half the neutral bar and the rest in the other without checking which neutrals were for which circuit.

 

[brightspark]

The other one that used to happen is when someone decided that they would modernize there own consumer unit and swap a neutral over on a dual rcd board and then find the rcd would not hold in.

and also trip when there's a borrowed neutral on a 2 way light over 2 circuits

 

[sparkysy]

This made me laugh having read @brightspark earlier comments.