This has happened when the client's room wasn't ready on time, he invited me to walk to the desk with him. The hotel had a price differential between the double for 1 guest and a double for 2 guests, put us really on the spot. I do advise always booking for two incase of any such problem.
Another time a horrendous accident on the motorway meant a client was at check in with me, they popped him in as my husband. I knew him fairly well, well enough that it wasn't a problem we laughed about it afterwards. Anyone else in different circumstances I may be iffy about.