Only three sorting criteria can be selected on the query edit screen.
With code before r16390, only three items could be saved as data according to screen specifications.
!{width: 50%; border: 1px solid #ccc}edit_query.png!

However, since "slice(0,3)" was changed to "slice!(3)" at r16390, the following result is returned now.

# slice(0,3)
sort_criteria = [['priority', 'desc'], ['tracker', 'asc'], ['priority', 'asc'], ['id', 'asc'], ['project', 'asc'], ['subject', 'asc']]
p sort_criteria.slice(0,3)
=> [['priority', 'desc'], ['tracker', 'asc'], ['priority', 'asc']]

# slice!(3)
# The number limit is not made
sort_criteria .slice!(3)
p sort_criteria 
=> [['priority', 'desc'], ['tracker', 'asc'], ['priority', 'asc'], ['subject', 'asc']]

I think it should be restricted to only three from the beginning like the case of "slice(0, 3)".
The attached file is a patch for fix.