ERRATUM Walks in the quarter plane: Kreweras' algebraic model There is a shameful mistake on the second page, in the introduction. The number of square lattice walks of length 2n confined in the first quadrant, starting and ending at the origin is b(2n) = 1/( (2n+1)(2n+4) ) binomial(2n+2,n+1) ^2. That is, the term (2n+2) must be replaced by (2n+4). This number is the product of the Catalan numbers C_n and C_{n+1}. In [20], the number b(2n) is written as a sum of 4 terms, each of them having a combinatorial explanation. A (recursive) bijective explanation of the Catalan product appears in Cori, Robert; Dulucq, Serge; Viennot, Gérard Shuffle of parenthesis systems and Baxter permutations. J. Combin. Theory Ser. A 43 (1986), no. 1, 1--22. A more recent, non-recursive bijective proof was recently found by Olivier Bernardi (http://www.labri.fr/bernardi/), whom I thank for having spotted this mistake in my paper.