Hopefully none of us will make this mistake, at least not so expensively!
http://www.bbc.co.uk/news/world-europe-27497727
Measure twice, cut once. ;)
When new "Electrostar" type EMUs were introduced to the former LB&SCR lines, no one thought that the additional air con and load would overload the 3rd rail system. You have never seen so many transformer/rectifier substations erected in such short time!
That also happens with the Pendolinos, certain parts of the network you can only have one at a time!
Not to mention the LMS sending and L&YR loco down the Manchester Buxton Derby line and wiping out all the canopy edges back in the 20s...