Search in a string

Below is the described substring searching algorithm:

def contains(text, pattern):
    for i in range(len(text) - len(pattern)):
        found = True

        for j in range(len(pattern)):
            if text[i + j] != pattern[j]:
                found = False
                break

        if found:
            return True

    return False

This version contains a mistake. Select all pairs of strings (the first is text, the second is pattern), for which the algorithm above gives an incorrect output.