New to the fertilising lark...

What would happen if I was to overfeed, by misreading instructions?

I havent I am just curious.

Depends on what sort of fertiliser, by how much you mis-read the instructions, and how often you repeated the mistake!   

Generally speaking there's quite a wide degree of tolerance - many of us get away with using a Standard British Handful of almost anything sprinkled over a Standard British Square Yard(-ish) !! 

I have heard of scorching plants by overfeeding them, but another thing is that you are just pouring your money away as your plants will never use the food. 
And on a large scale the surplus fertilizer ends up in the ground water and rivers polluting them.