If you create a block, and then undo, the model space remains as it was, but there is now a new consecutively named model in the Block library tool (e.g. "Block_3").
I can see this as either a bug or a feature. A bug if you don't like cleaning out the block library, but a neat way to create a block without having to commit the object you're working on to becoming a block -- maybe even a kind of partial save.
TCAD V16 Design