By my own estimation based on errors reported to me through CR or that I've discovered myself, shows which I work on completely alone on have an error rate of around 0.05% per line.
I.e. I tend to have a typo get through 1 out of every 2000 lines. At an average of 300 lines an episode, that's one mistake every 7 episodes or so.
I consider this reasonable enough accuracy for simulcast translations on the cheap. A secondary editor or qcer can probably cut that down by another 50% or so to an error every 13 episodes or so.
Getting it down farther than that requires at least 2 independent qc passes and I'd consider only necessary for DVD or blu-ray subs where mistakes cannot be fixed.
The amount of qc you need or want is entirely dependent on the quality of your original staff and the quality you want to release. It's not a "mistake" to have too much or too little qc, it's simply a conscious choice about how to work.
Just as it's not a "mistake" to release in avi instead of mp4.
That's a choice that depends on the audience and venue you are trying to reach.
(On the other hand, doing h.264 in avi is a mistake
).