If you build the contraption shown in the first picture below, and switch the lever, it will look like it duplicates the slimeblock and an iron block. The duplicate slime and iron block are ghost blocks.
There have been several other bug reports about ghost blocks. The reason I make another bug report is, that this particular method to generate ghost blocks has only started working in 1.9-pre 1. It didn´t work in 16w07b and before. So this is not supposed to be a bug report about ghost blocks in general, but instead a bug report about this particular method to create ghost blocks, which is a new bug and not a duplicate of any bug that existed before 1.9-pre 1.
Now I don´t know why the piston behavior was changed from 16w07b to 1.9-pre 1. If there´s no significant reason why it was changed, then the simplest way to fix this bug, would be to revert the changes in piston behavior, that were made from 16w07b to 1.9-pre 1. This way this ghost block generation method could be fixed very quickly and efficiently before 1.9 gets released.