From the sudoers manpage: 
[...] sudo will read each file in /etc/sudoers.d, skipping file names 
that end in ~ or contain a . character to avoid causing problems with 
package manager or editor temporary/backup files. [...]
This mean if you have a Unix user like "lars.windolf" you do not want to create a file 
/etc/sudoers.d/lars.windolf
The evil thing is neither sudo nor visudo warns you about the mistake and the rules just do not work. And if you have some other definition files with the same rule and just a file name without a dot you might wonder about your sanity :-)