Constant STALE_GROUPS_HINT 

const STALE_GROUPS_HINT: &str = "The `groups` metadata is stale: the grouped frame was \
     likely subset or reordered without dplyr loaded, which keeps the old `groups` attribute. \
     Regroup it with dplyr::group_by() (with dplyr loaded, `[` regroups by itself), or read just \
     the grouping declaration (DataFrame::group_declaration) and group the current rows.";

The cause and the fix that the stale-.rows errors (DataFrameError::GroupIndexOutOfRange, DataFrameError::GroupRowUncovered, DataFrameError::GroupRowDuplicated) append to their message.