If you have managed a maintenance or project budget you will have experienced the pressure that unplanned maintenance causes in trying to meet budget. Often a lot of planning has gone into planned activities that are put on hold, whilst money is thrown at unplanned interruptions. It is easy to compile a budget for scheduled activities. But allowing for the unplanned- how do you do that. Most people use experience and make some contingency or have a "bucket" they can draw from. Random failures quite often fall into the category of unplanned failures. But a random failure leading to "Fix on Failure" is a legitimate strategy where monitoring or inspection activities, and redesign/modifications are not applicable.
To predict the amount of resources and budget required for random failures, describe the likely failure rate, and then use a Simulation engine to predict the number of failures expected in a given time frame. You may not know exactly which random failure will occur or when, but you will have resources available to attend to them, without having to "rob" the scheduled work budget.