Here's an awesome picture of a mucus plug pretty much completely intact. This mom was 9 centimeters when this came out. Her membranes were intact.
Photograph by Barbara E. Herrera, LM, CPM
You can imagine that if you "lose your mucus plug" pre-labor, it would be slimy and runny and you might have to wipe a couple three times. It also tends to keep coming out over a period of time, stretching out a little at a time over a couple of days even sometimes.
And just like when we have a cold and the body keeps making mucus in our nose, the body keeps making mucus in our vagina, too, so we technically could have more mucus plug again, though a complete other plug doesn't often happen. Mucus, yes. Plug, rarely.
Thanks to the mom for her permission for letting me use her beautiful plug to share with others. It's taken me years to get a picture of an intact mucus plug. I've held onto this for a couple of years so I could remove any hint of who the mom might be (even though I do have her permission). I came across it with the amniotic sac pic and here you go!