Suppose we have the following code:

    from heapq import merge

    def iterable():
        lst = range(10)
        for i in xrange(20):
            yield lst[i]
        
    it1, it2= iterable(), iterable()

    print list(merge(it1, it2)) # no IndexError
    #output is: [0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9]


The reason is that in heapq.merge http://hg.python.org/cpython/file/7c18b799841e/Lib/heapq.py#l372 try-catch clause for IndexError is too broad

    while 1:
        try:
            while 1:
                v, itnum, next = s = h[0]   # raises IndexError when h is empty
                yield v
                s[0] = next()               # raises StopIteration when exhausted, 
                _heapreplace(h, s)          # restore heap condition
        except _StopIteration:
            _heappop(h)                     # remove empty iterator
        except IndexError:
            return

s[0] = next() also may raise different kinds of exceptions including IndexError which will be silently suppressed. 
For example, this loop can be rewritten as

    while 1:
        try:
            while 1:
                try:
                    v, itnum, next = s = h[0]   # raises IndexError when h is empty
                except IndexError:
                    return
                yield v
                s[0] = next()               # raises StopIteration when exhausted, 
                _heapreplace(h, s)          # restore heap condition
        except _StopIteration:
            _heappop(h)                     # remove empty iterator