I saw a fun talk at a conference in December where Jeff Lichtman showed us some of his processed data from his 5x5x30 nanometer resolution mouse brain tissue electron microscope scans (of which probably a 30 micron wide cylinder was actually properly annotated). He mentioned that only 40% of the things-that-were-clearly-synapses in the annotated dataset were the large spine-based synapses that have been well studied, and that there were a couple cells in there that he had no idea what they were at all.