It is not sufficient to simply take reliability numbers at face value. MTBF numbers are rapidly spiraling higher. Meanwhile, the device is typically overtaken by the next generation of the technology before the reliability claim can be substantiated empirically. MTBF is intended to predict the rate of device failure, not provide a guarantee of continuous trouble-free performance. Unfortunately, MTBF can not predict when the failure will occur.
Failure occurs randomly.
It is important to be aware that differences exist among the various expressions of reliability. Ask the questions enumerated previously before comparing reliability numbers from more than one vendor. Their regular assessments of calculated and empirical data are periodically provided to engineering and manufacturing to be factored into continuous process and reliability improvements.