If you use SnipMate (highly recommended) you may frequently end up with this situation. Let’s say you are extracting a method in a ruby file, so you need a new method but you intend to copy some lines from the original. You might start by typing def<Tab>my_new_method<Esc> with the following result:

def my_new_method
··
end

But now when you paste your original lines you’ll have to go back and delete the SnipMate-generated line with its trailing whitespace. I’ve never wanted to keep a line with trailing whitespace when pasting over it, so I use the following in my .vimrc:

" Paste over lines with whitespace (common when using snipmate)
nnoremap <expr> p getline('.') =~? '\v^\s+$' ? "\"_ddP=`]" : "p=`]"

Essentially this says upon (p)ut if the line only contains trailing whitespace, dump the line into the black hole register and (P)ut. The extra characters after the put command are just there to auto-format, so if you are pasting code that was indented differently it will adjust based on its new context.