The Half That Vanished

def merge(left, right):
    out = []
    i = j = 0
    while i < len(left) and j < len(right):
        if left[i] <= right[j]:
            out.append(left[i]); i += 1
        else:
            out.append(right[j]); j += 1
    out.extend(right[j:])
    return out

def merge_sort(a):
    if len(a) <= 1: return a
    mid = len(a) // 2
    return merge(merge_sort(a[:mid]), merge_sort(a[mid:]))

merge_sort sometimes returns a list shorter than its input. What is the shortest input that triggers it?