Ref T10688. When you have a translation like "I saw %s dog(s)", you can provide multiple variants in the translation file that are selected based on the argument's plurality or gender.
However, if you pass something which is not a number and not gendered, we can't use it to choose a variant (we have no idea which variant we should select for a phutil_tag() or arbitrary string).
An example of bad variant data is:
'%s touched %s %s time(s).' => array( array( '%s touched %s once.', '%s touched %s %s times.', ) )
This data intends to vary on the third parameter (the number of touches) but actually varies on the second parameter (the touched thing).
Unless that second parameter is a number or a person, this data is incorrect and tries to vary on a non-gendered, non-plural value.
The correct data uses an extra array to vary on the third parameter instead of the second parameter:
'%s touched %s %s time(s).' => array( array( array( '%s touched %s once.', '%s touched %s %s times.', ) ) )