Let's put this in non-English terms.
We have a deepwater basketweaving specialty area within our underwater basketweaving department. Deepwater basketweaving has recently (within the past 20 years) become way more developed as a specialty; before that, interested underwater folks used to teach the class. Or maybe deepwater basketweaving used to be a bigger area, but student interest in other areas has led us to drop our faculty coverage down.
At any rate, it's a specialized area, but any basketweaver knows a little about it, so when we can't cover the intro classes with our TT faculty, the chair calls an adjunct at the last minute and says, basically, hey, want an extra course? And the adjunct, happy for the course and usually eager to please, says sure. The chair breathes a sigh of relief and moves on to other explosive problems.
And because the adjunct is eager to please, s/he gets good student evaluations for the course. And the next time, well, the adjunct is experienced in the field, and so teaches it again, and again, and again. Each time, the adjunct puts in a good effort (because adjuncts work their tails off), coming to deepwater basketweaving from his/her specialization in brackish water basketweaving, or reed preparation.
And eventually, maybe we hire a new deepwater specialist. Or maybe the students feed into the open water basketweaving courses. But at any rate, someone begins to notice and then, after a while, to complain sotto voce that the students really aren't getting what they should be getting in that introductory level course. It's not being taught as a true deepwater course.
Or maybe we realize that we need a TT line in deepwater basketweaving, and our beloved adjunct applies.
In whatever way, we come up against the hard fact that an adjunct we really like, one who has worked his/her tail off for us in all sorts of ways, really isn't doing the job with the deepwater class, but expects to teach it again, or to be looked at seriously for the TT position. And the adjunct has pals. Or the adjunct's spouse teaches in the department (or is an administrator). Or the adjunct got his/her BA or MA here and is geographically limited. So there are people who really want to go to bat for the adjunct. (And always, in my experience, the adjunct is a straight, married, white person who's deeply connected to our area, who looks and talks just like our students, makes jokes about having gone to the local high schools.)
We've been depending on this person we shouldn't have depended on in the first place for this course because it was easy at the time. And a lot of people remember when deepwater wasn't even a specialty, and people from all over covered that course.
So we have a conflict between being jerks to the adjunct, who we've been exploiting all along, or doing a disservice to another colleague (who has to deal with inadequately prepared students) and doing a disservice to our students, who don't even realize they're being cheated.
If you're friends with the adjunct but think we need a real deepwater specialist, what do you do? And what if deepwater specialists aren't too common, and we're going to have a hard time recruiting one because we pay poorly and are far from deepwater research opportunities?