A Lewiston man said he is considering legal action after finding a knife in the store-bought cake he purchased for his son’s second birthday party. Nathan Bibeau said that as the family was slicing into the Elmo birthday cake bought at the Auburn Walmart someone spotted a small metal paring knife. Bibeau told WGME-TV that he was ‘‘pretty disappointed’’ and that he called Walmart immediately. Walmart apologized and offered a refund. A corporate spokeswoman said the knife incident ‘‘should not have happened,’’ and the retailer was eliminating use of the knives at store bakeries nationwide. Bibeau said that his son’s birthday party was ruined.