Common Mistakes in Blocks with variants

The diagram below shows the typical files needed to render a simple block with variants (BlockMT01).  The most common errors related to variants are highlighted. 

Note that for blockstates with multiple properties the order in the blockstates file  is important, i.e.

colour=red,facing=north 

won't match
facing=north,colour=red

For the list of symptoms that these errors produce, see here.

For common mistakes in Blocks that aren't related to variants, see here.
For common mistakes in Items, see here.
For common mistakes with filenames and directory structure, see here.