I have actually had this happen on a number of occasions but not because I couldn't fit them in! The client was having their wax appointment and asked me ! Some clients are just plain cheeky and you know who they are.
However the nicer clients really just don't think it would be cheeky to ask. They must think we don't have to pay for it! You wouldn't go to a cafe and use their cups to pour out your vacuum flask of coffee would you? If you had gone in for cakes only lets say.
Such is the variety of personalities that come as part and parcel of our jobs, remember who they are and make sure to upsell whenever you can. Ensure they pay in full ,no discounts for loyalty and you'll get over their blatant cheek!
If it's a client that does it regularly then that is different. Keep your acetone out of sight when they arrive and ask just say " Oh dear, I have run out I need to get some on order!" They will soon get the message.
What did you do in your situation?
Best of luck