In all engineering, the is the FS variable. It's the Factor of Safety. This value is used to account for "unknown" variable forces and for redundancy. It also somewhat accounts for "human error" and discrepancies during construction. Whatever this value is determined to be (for example, it's often 6 for structural steel bridges), ALL forces are multiplied by it during design. I don't know what the FS is for slide/coaster design, but if part of the boat did indeed leave the track, it seems that either the initial forces were underestimated, or the FS was not high enough to account for variable forces such as wind, riders shifting, hydraulic gradient, etc.
Or perhaps all of the ride engineering was correct, but human error came into play with something such as the restraint not being properly fastened. Even so, if the ride had so many dangerous variables, it seems the restraint should have had some redundancy.