Forgetting to Perform an Obligatory Action of the Prayer
Maintaining proper order between the obligatory actions of the prayer (e.g. standing, bowing, prostrating) is also obligatory. The validity of the prostration, for example, is dependent on its following the bowing. If one prostrated without bowing first, then the prostration would be considered inconsequential and it would need to be repeated. One would return to the standing position, perform the bowing that one forgot, stand up again, and then return to the prostration.