Abstract: Machine for folding a web (3) in a zigzag manner includes a transport mechanism (2) for the web (3), a receiving device (5) at the bottom for collecting the folded web, a folding mechanism (4) mounted between the transport mechanism (2) and the receiving device (5) and a detection device (19,20) to detect improper folds mounted under the folding mechanism (4), wherein the machine includes a device arranged to move the web (3) backward in the folding mechanism (2) and a control device (22) connected to the backward moving device and the detection means (19,20), so that, at least upon a first detection of an improper fold, the web (3) is moved backward in the folding mechanism (4) by said backward moving device over such a distance that the improper fold is entirely unfolded and so as to afterwards allow again for the normal forward movement of the web (3) through the folding mechanism (4).