Let's say you want to make use of ignorecase as that's what you'd like when searching with / and ?. However you then realise how oddly pervasive this setting is in that is applies to things like :s. Rather than inserting "\C" into every pattern you think you'll use use an autocmd and so some up with something like the following config.

set ignorecase

autocmd CmdLineEnter : set noignorecase
autocmd CmdLineLeave : set   ignorecase

Only to find it doesn't work, ignorecase still seems to be in effect when using :s. This is because CmdLineLeave fires not only before you truly leave the command-line but also before the command has executed.

Here's a snippet to overcome this in a generic fashion by using a timer.

let [s:ignorecase, s:smartcase] = [&ignorecase, &smartcase]

function! s:RestoreCase(ID)
    let [&ignorecase, &smartcase] = [s:ignorecase, s:smartcase]
endfunction

augroup SaneCase
    autocmd!
    autocmd CmdlineEnter : set noignorecase nosmartcase
    autocmd CmdlineLeave : call timer_start(100, '<SID>RestoreCase')
augroup END

The far reaching effecting of ignorecase annoys me enough as it is but CmdLineLeave firing before executing the command really takes the biscuit here.