In my survival world, I spawned above a stronghold. I went to find the End Portal, and when I did, I broke the silverfish spawner. But the Monster Egg below had a silverfish jutting out. This might actually be the fact that the spawner did not get rid of the silverfish inside of it.

Update: Since everybody thinks that this is intended, all you have to do is reproduce it, and all you have to do is find a stronghold and break the silverfish spawner, and no doubt it will happen.