Summary
When $filter is used against an array of custom objects, I received this console error in 1.5:
Error: key.charAt is not a function
deepCompare@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.11/angular.js:20587:16
deepCompare@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.11/angular.js:20587:42
deepCompare@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.11/angular.js:20587:42
createPredicateFn/predicateFn@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.11/angular.js:20562:12
filterFilter/<@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.11/angular.js:20526:12

And in 1.6:
Error: key.charAt is not a function
deepCompare@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.1/angular.js:20564:16
deepCompare@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.1/angular.js:20564:42
deepCompare@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.1/angular.js:20564:42
createPredicateFn/predicateFn@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.1/angular.js:20539:12
filterFilter/<@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.1/angular.js:20503:12
anonymous/fn@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.1/angular.js line 15156 > Function:2:489
regularInterceptedExpression@https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.6.1/angular.js:16264:55

Steps to repro:
I saw this bug when iterating over a goog.structs.Map, and noticed that key was actually a number. Sorry, I can't reproduce this without figuring out how to pull in closure on plnkr. And it is still hard for me to believe that a for (var key in obj) loop can produce non-string keys, so I don't know how else to possibly reproduce it.

The best I can say is that this definitely was an issue on >= Angular 1.5 on Firefox (but not Chrome). I'm not sure about IE.

Possible fix:
checking for typeof key === 'string'
here: https://github.com/angular/angular.js/blob/master/src/ng/filter/filter.js#L229