Similarily, unbelievable => unbelievable ~ umbelievable. The sounds "m" and "b" share a labial (i.e. lip) feature, whereas "n" and "b" do not.
There's also, supposedly => suppozedly => suppozubly (-ub as in tub). /s/ is pronounced as [z] intervocalically (i.e. between two voiced sounds). /e/, a lax vowel, is neutralized to a wedge, for which I've used "u" to symbolize. "d" changes to the sound "b" by a process called progressive assimilation. That is, "b" and "p" share a labial feature.
supposedly ~ supposebly