Some people have reservations for a restaurant, while others have reservations about a restaurant. A popular joke has been cited in print since at least 1975:
 
RESTAURANT HOST OR HOSTESS: Do you have reservations?
CUSTOMER: Yes, but I’ll eat here anyway.
 
Versions about hotel reservations were cited in print in 1992 and 2012.
 
   
19 May 1975, Honolulu (HI) Star Bulletin, “Dave Donnelly’s Hawaii, pg. A-4, col. 1:
Which reminds us of the wag who, upon entering Sam’s Place was asked, “Do you have reservations?” “I certainly do,” replied the gent, but we’re going to eat here anyway.”
       
Google Books
The Logical Accessory to Critical Thinking:
Evaluating claims and arguments in everyday life
By Brooke Noel Moore and Richard Parker
Palo Alto, CA: Mayfield Publishing Company
1986
Pg. 43:
When the head waiter asked whether she had reservations, she said, “Yes, but I’m going to eat here anyway.”
     
Google Groups: alt.tasteless.humor
Sick Humor from Funny Pages #34, Jul ‘92
Joe Workman
1/12/96
Hey, I publish a sick joke newsletter, Funny Pages.  Here’s an old issue—there’s more of ‘em at:
http://www.erinet.com/jworkman/fphome.html
(...)
The newlyweds showed up at the hotel and asked for the honeymoon suite: “Do you have reservations?”
“Only one, she won’t take it up the ass.”
 
4 March 1998, Shreveport (LA) Journal, “Only the Good Die Youngman” by Tim Greening, pg. 9A, col. 3:
“Last time I came here, the maitre d’ asked, ‘Do you have reservations?’ And I said, ‘Yeah, but I’ll eat here anyway.”
(Crediting the joke to comedian Henny Youngman, 1906-1998.—ed.)
 
Twitter 
Paul R Brown‏
@paulrbrown
Trotted out one of my Dad’s chestnuts at lunch today.
“Do you have reservations?”
“Yes, but I’ll eat here anyway.”
3:52 PM - 7 May 2009
         
Google Books
The Mammoth Book of One-Liners
Edited by Geoff Tibballs
London, UK: Constable & Robinson Ltd
2012
Pg. ?:
Hotel clerk: “Do you have reservations?”
Guest: “One or two, but we’ll take a chance anyway and book a room for the night.”
   
Twitter
Rick Aaron‏
@RickAaron
Dad’s joke - every hotel:
Dad: I’d like a room for tonight.
Clerk: Do you have reservations?
Dad: Yes….but I think I’ll stay here anyway.
10:47 PM - 18 May 2013
 
Reddit—Jokes
Reservations
submitted December 15, 2015 by maganda1220
A couple walk into a nice restaurant on Friday night, hoping to get a table. Upon seeing the couple, the host asks the couple, “Do you have reservations?” The wife replies, “Yes, but we’ll eat here anyway.”
 
Reddit—Jokes
“Yes, I have reservations, but I’ll eat here anyway.”
submitted April 19, 2016 by ticktockaudemars
my dad to every hostess ever
COMMENTS
JokeExplainBot
Reservations can mean a reserved table or doubts.
 
LicenceToBill
I heard this kind of in reverse from Tim Vine.
I was checking into a hotel and the receptionist asked me “do you have a reservation?” I said “Yes…I’m a little worried about the price.”
 
Twitter
GoDeep,I’llLookForU‏
@MattSchultz13
Replying to @Xzeander @ife_luv12
Hostess: Do you have reservations?
Me: I do but I’m gonna eat here anyway.
10:46 AM - 22 Aug 2017
 
Twitter
ReallyGamerDude‏
@ReallyGamerDude
Dad Joke Of The Day: Guy walks into a restaurant and the hostess asks: “Do you have reservations?” The guy replies: “I do, but I’m going to try your food anyway.” #dadjokes #puns #verypunny
10:55 AM - 17 Nov 2017