A Lactobacillus plantarum strain, CECT 4645, was found to have insertions of a sequence (985 bp in length) at least in eight loci in its genome. The prototype copy (Lp1) of this insertion sequence (named ISLpl4) has one open reading frame encoding a putative protein that is 292 amino acids in length with significant levels of similarity with IS982 family transposases. Perfect 16-bp inverted repeats were found at its termini. Upon transposition, generates 8-bp direct repeats of the target sequence, but no consensus sequences could be identified at either insertion site. The ISLpl4 pattern changed over many generations on the CECT 4645 strain. This finding strongly supports our hypothesis that ISLpl4 is a functional element in L. plantarum. Some of these elements may be cryptic, since point mutation or 1-nucleotide deletions were found in their transposase encoding genes. ISLpl4 copies have been detected in Leuconostoc mesenteroides, Oenococcus oeni, and Lactobacillus sakei. An ISLpl4 copy of O. oeni contained a +1 nucleotide insertion on its transposase encoding gene and, by using an experimental system, we were able to demonstrate that this specific sequence originates a +1 programmed translational frameshifting. Although the frameshifting process reported here operates at a low rate, this description might represent the first case of a functional +1 frameshifting among IS.