I have a leaky join between guttering and the downpipe bit. In the photo it's the bit that's all black / green with the obvious signs of sealer that used to be there. It's leaked on and off for years.

Rain gutters take a lot of abuse: rushing water during rainstorms, exposure to weather extremes, heavy loads of snow in the coldest climes, clogs of fallen leaves, and the general ravages of time.