"er" after "r" sounds.

Syllabic "r" tends to be elided in my speech when occurring immediately after an [r\] sound.

"mirror" - [mIr\]
"terror" - [tEr\]
"horror" - [hOr\]
"error" - [Er\]

This leads to sentences starting with "there are" to sound as though the "are" were not present:

"there are a lot of them" [DEr\ @ lA4 @v DEm].

Note I often tend to pronounce the [@`] in "horror" for obvious reasons.

I do so only in the word "mirror", which I pronounce ["mI:R]

I think most people do that -- just hold the first R a little longer to indicate the second R. So "terror" kind of sounds like "tear" (as in ripping, not crying) with the R held a lot longer -- "tearrrr".

This thing doesn't occur in my speech, because I have the Mary-merry-marry, serious-Sirius, hurry-furry, and Tory-torrent distinctions, so homophonies like "mirror-mere" or "terror-tear" aren't natural for me. I have:

"mirror" ["mI.r\@`], cf "mere" ["mI@`] or "nearer" ["nI@`.@`]
"terror" ["t_hE.r\@`], cf "tear" ["t_hE@`] or "tearer" ["t_hE@`.@`]
"horror" ["hQ:.r\@`], cf "whore" ["hO@`] or "sorer" ["sO@`.@`]
"error" ["E.r\@`], cf "air" ["E@`] or "airer" ["E@.`@`]

In each of those four words in question, I think the vowel that I use is opener than that used by most North Americans, and I would also consider the syllabification to be different. I'm familiar with the pronunciation of "mirror" as ["mI@`] from speakers from other parts of the country, but it doesn't seem natural to me. Also, there have been a couple times when I've heard people say something like ["hO@`.@`] or ["hO:r\:], and I've had to do a double take to realize that they were saying "horror".

Does this mean you go to "harror" movies, Lazar?

No, because even though I have the Tory-torrent distinction, I also have the father-bother distinction. So I would say:

horror - ["hQ:.r\@`] or "haw-rer"
harrer ("more har") - ["hA@`.@`] or "har-er"
sorer - ["sO@`.@`] or "sor-er"

But I think most people from NYC or Providence would go to "harrer" movies.