There needs to be another dimension in this model. In building maintenance, there needs to be an inspection to ensure the maintenance was performed properly. For example, you may replace a cracked roof shingle as a do-it-yourself preventive maintenance activity, but not do it correctly and actually cause a leak that was not there when it was only cracked.

In software, you need to have code reviews, unit and regression testing, or you can fix one minor bug only to introduce a catastrophic fail.