The Problem I had too many times now is that if Im trying to delete a huge quantity of Dirt with a enchantent shovel (efficiency V fortune II and unbreaking III) some blocks seem to not despawn but are invisible so that I get stuck in them, when Im reloading the game the Blocks wich I was getting stuck in are spawned again!
Ah yes, this is referred to as ghost blocks. They are really very annoying but are fixed in the 1.13 snapshots