Affects Version/s: Minecraft 16w40a
Fix Version/s: Minecraft 16w41a
Vindication illagers with the Johnny tag set to true will attempt to attack armor stands. This is of course, fruitless, because they cannot take damage. The reason for this is almost certainly because armor stands extend "living mob," and Johnny vindicators attack living mobs.
Note that withers, who also attack living mobs, are also affected. Other bug reports describing this issue were assigned as duplicates of MC-1673, though it seems to be to be a separate issue – mobs that attack other mobs try to attack armor stands.
Edit: Never mind, the report for this behavior on withers is