The Leverkusen forward initially put his hands on his head and looked confused when referee Felix Brych awarded the goal while his team mates surrounded him to celebrate.
The decision is likely to renew the arguments over the Bundesliga's refusal to use goal-line technology, especially as it was the second time this season that hapless Hoffenheim were on the wrong end of a contentious decision.
Leverkusen officials admitted the goal should not have been given and Hoffenheim's football director Alexander Rosen said they would protest.
"We will definitely do that," he said. "The Leverkusen players did not celebrate. It's scandalous."
Referee Brych, looking disconcerted, left the field to a resounding chorus of jeers at the end.
"I had a slight doubt, but the reaction of the players was clear," Brych told German media. "Nobody said to me that the ball did not go in. For me, it's not a great situation to have given a goal that didn't exist."
Kiessling looked bemused as he told Sky Sports Deutschland: "I initially thought that the ball was not in, but then everyone came over to me to celebrate."
LITTLE UNPLEASANT
Leverkusen coach Sami Hyypia added: "I saw after the game that the ball went just past the post. During the match, I celebrated, but naturally it's a little unpleasant to win like that."
A FIFA source said that goal-line technology would have detected that the ball had not gone in.
Hoffenheim's misery was compounded with a bizarre sequence of misses after they were awarded a penalty shortly after falling two goals behind.
Roberto Firmino saw his shot saved by Bernd Leno. Firmino then fired the rebound against the post and the ball fell to Omer Toprak, who also hit the same upright.
They finally pulled a goal back with a Sven Schipplock header.
Sidney Sam had given Leverkusen, who are top of Bundesliga with 22 points from nine games, a first-half lead with his seventh goal of the season.
They are two points ahead of Bayern Munich who host Mainz on Saturday, while Borussia Dortmund are a further point behind and host Hanover 96.
It is the second time in this campaign Hoffenheim will feel aggrieved about a match-changing decision.
They had a goal controversially disallowed on the opening day of the season against Nuremberg when Kevin Volland skilfully chipped the ball over goalkeeper Raphael Schaefer and into the goal.
Replays showed that the ball landed well behind the goal-line before bouncing back out and into play, Nuremberg players turned away in disappointment and Volland was on his way to celebrate when referee Thorsten Kinhoefer waved play on. Hoffenheim were winning 1-0 at the time, but were held 2-2.
Germany has ruled out adopting any goal-line technology until at least 2015. It has also rejected the additional assistant referees who are positioned on each goal line and are used in European competitions and Serie A.
The incident brought back memories of Thomas Helmer's infamous phantom goal for Bayern Munich in 1994 when he scrambled the ball wide only for the referee to rule that it had gone in.
Bayern went on to win the match against Nuremberg 2-1 and, although the result was annulled by the Bundesliga, they won the replay 5-0.
(Writing by Brian Homewood; Editing by Toby Davis)
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