When I saw the title of this thread, I was about to say, not another one of these again... But yeah, it would suck if they did put an MCI on a local route, but as others have said I've seen plenty of MCIs with local bus runs on them just for the hell of it. This one B/O out of Ulmer Park loved flying down Fifth Avenue as a B36 to Avenue U, which I have to say looked pretty damn cool. Then I'd take my express bus and see him parked over by 23rd st relaxing and such later on. So as others have said, it could've been a number of things.
The one thing I have noticed is that

Bus must be much more strict than NYCT with some things because I never see express buses in Riverdale signed up as something they aren't and the drivers are adamant about enforcing the rules. No getting off at the corner or any of that like the SI express buses unless they are permitted to at night (Request A Stop) or the stop is actually located at the corner. They don't like folks standing either I notice. Last weekend, my BxM2 had to do a detour because of some incident on 7th Avenue by 42nd street, so we had to detour to 5th Avenue and I'm thinking oh cool, I can get off at 34th & 5th since I was heading to Whole Foods, but the driver said he couldn't let me off. I have however been let off during detours on the X10 though and other SI express buses as well. His buddy or whatever (another B/O) who was with him had the nerve to ask me (albeit politely) to sit down. I gave him a look like who in the hell are you? Isn't that the B/Os job, but I thought about it and understood why so I cooperated even though I did give him a dirty look. lol
The thing is in Riverdale esp. folks (including moi) will write in like crazy if there are problems like with my bus stop, so they really have to be careful. There's also the issue of folks that work for the

that live in Riverdale. I found out that the dispatcher came specifically for our stop the other night because one of the guys who waits at my stop with me works with Joe Lohta and he like myself made a stink about our stop being passed on several occasions (he had actually been bypassed far more times than I had and in fact told me his bus bypassed him as he was talking with Joe Lhota at the stop, but he didn't want to get the B/O in trouble lol), and so far since Monday all buses have been making the correct stop, so I haven't had to make any additional phone calls or anything to my contact at 347 Madison. Pretty sweet.

He actually didn't want to say anything, but clearly there was a problem so he had to.