Dear Subordinate,

I can no longer contain myself.  I must let you know that I feel your same day request for me to write you a letter of recommendation is quite unacceptable.  I would propose that you consider asking me at least a week in advance in the future.  If you do not, I will be required to dispense an unintended undesirable notification of investigatory conference or a voodoo curse. All this may come as news to you; however, for what it is worth,  lack of planning on your part does not constitute an emergency on my part. I implore you to a) not take this personally, b) let us never speak of this again, and c) seek help from another colleague.

Sincerely,

Your Stabby Supervisor