I found a natural vein of silverfish stone under an extreme hills and as I was fighting them more spawned from the air where the block previously was. 8 blocks broke and 22 silverfish spawned before I died.

I expected the Silverfish to stop spawning after all the silverfish stone blocks were broken.

The Silverfish continued to spawn after all the silverfish blocks were broken.

Steps to Reproduce:
1. Find a vein of silverfish stone
2. Spawn silverfish
3. Hit the silverfish a bunch