sing-along » sing-a-long
In this eggcorn, the reanalysis is evident only in the way the word is punctuated. The original form, sing-along, is quite transparent, so I suspect that the reanalysis is not motivated by the sense of the word so much as by analogy with “V-a-N” constructions such as Rent-A-Wreck, rope-a-dope, and whack-a-mole.
I’ve labelled this one as “nearly mainstream,” because the reanalyzed form seems to be very common. As of this writing, the query “sing-a-long gets approximately a third as many hits on Google as the query “sing-along”, although it should be noted that, since Google treats hyphens and spaces as interchangeable, the hit counts will include things like “I run out of air if I have to sing a long note” and “Feel free to sing along.”
Another confounding factor here is an outfit called Sing-a-long-a, which puts on sing-along screenings of the film The Sound of Music (”the ultimate communal nun-based karaoke“). These events go under the official title Sing-a-Long-a Sound of Music, but the second “-a” is often omitted by reviewers, as in this article by Beth Nissen at CNN.com.