I received an enquiry about our service late into the day yesterday, about 5 minutes from finishing for the day. So I fired off the standard "you might be able to pay for this out of your [company policy redacted] allowance, or alternatively speak to your line manager to find out whether this can be approved by your department budget" response. Job done, home time.
Got in this morning, boss comes in and draws my attention to the person's job title in their signature line - Managing Director for [countries redacted].
I haven't had a read receipt from him so I'm assuming he hasn't seen the initial message, but I'm still kicking myself. Luckily, my boss isn't one to drag me through the wringer for something like this (which could just as easily have happened to my coworker who's been here far longer than me), so we set into immediate preventative damage control, and as far as he's concerned, that's a lesson learned and we know how to approach this next time. I'm still feeling like a twat though.
And this, my dear GTers, is why you should read things thoroughly and not always stick to the script.