Conditional Incrementing Incorrectly

My function

int numSteepSegments(const int heights[], int arrSize) {

    int mile = 0, steep_count = 0;

    while (mile <= arrSize) {
        int height = heights[mile];
        int height_next = heights[mile + 1];
        if ((height + 1000) < height_next) {
            ++steep_count;
        } else if ((height - 1000) > height_next) {
            ++steep_count;
        }
        cout << heights[mile] << endl;
        cout << steep_count << endl;

        ++mile;
    }
    return steep_count;
}

is spitting out twice as many steep_count than it is supposed to.

With the array: 1200, 1650, 3450, 2800, 2900, 1650, 1140, 1650, 1200, and the arrSize = 9, what am I missing?

the cout's are:

1200
0
1650
1
3450
1
2800
1
2900
2
1650
2
1140
2
1650
2
1200
3
1746942513
4

What is that last value? It's obviously not in the array, and I can't see it belonging anywhere near the numbers I'm dealing with. What am I not seeing in my conditionals that's causing the wrong increments to steep_count?