In block_schema(), the 'block' table, the column 'delta' currently is a varchar(32).
It is quite easy to hit this limit. Especially if the block delta is composed from different parts for disambiguation.
E.g. mymodule-footer_product_categories (the block's module is "menupoly", not "mymodule")
Or #1512656: Block delta too long with some menus ("Better Jump Menus")
Or https://www.google.com/search?q=Data+too+long+for+column+'delta'+drupal+block&oq=Data+too+long+for+column+'delta'+drupal+block&sourceid=chrome&ie=UTF-8
Changing it from varchar(32) to varchar(64) would be an improvement already..