Amazingly enough, no technology that involves people is 'perfect'.
Some days, equipment fails. Encryption stuff seems to LOVE to fail (lose coding). It has this 'anything fooling with me must be trying to steal my code, ERASE it!' thing, I suspect, so a glitch in power can result in 'Code? What code?'.
Some days, the technicians just plain miss loading the new codes into a unit.
Some days, the user just plain bumps the code/clear switch, without noticing. (The radios in surveillance cars tend to be hidden under the seat or otherwise not in clear view).
Sometimes, ONE user's radio has fully failed encryption (can't send OR recieve encrypted), and everyone must go clear, or they have to send that unit home to base and not use them. Other times, for whatever reason, radios won't send encrypted, but do recieve it, and it ends up that everyone but them is encrypted. The radios 'switch' automatically, so everyone else can hear him just fine in the clear while they're encrypted.