* When stoppers are touched by [[module]]s touch stopper, a wooden [[sound effect]] is produced. hovever, it producing that sound does not need the stopper to be sent into in any extra sound effect group to produce that soundgroups.* Despite the existence of the sound effect, only the first stopper in the same level (the first in the Phys_FloorStopper group) is able to produce sound effects. Though this means that a hidden stopper without sound effects can be made, all stoppers that need sound effects in the same level should must be attached into the same object and be firstly sent into the Phys_FloorStopper group.* The [[guard rail]] for blocking the sandbags in the third section of Level 13 is actually a series of guard rails (grouped as Phys_FloorRails) and a stopper of the same model. The stopper is named as the standard "Floor_Stopper". Therefore, [[sandbag]]s produces the special impact sound on conpact contact of the guard rails; similarly, the guard rails that block [[turnstile]]s in the 4th [[sector]] is actually a stopper.