We happened to look next door at this home during an eavestrough project in Toronto and saw some very evident problems.
The eavestrough has an open end which dumps water onto the roof instead of channeling it to the lower eavestrough via a downspout.
Over the years the high volume of water near the end of this gutter has slowly eroded the surface of the asphalt shingles.
You can see the layers of newer, different color shingles that have been installed to probably stop a leak into their home.
This probably was not enough to fix the problem as it seems that someone poured a bucket of black tar down the roof out of desperation.
Never allow a high volume of water to flow down your roof like this or you can expect premature roof wear.
When hiring a contractor to work on your home ensure that they have a "do it right do it once" attitude just like Mr Holmes says.
Visit http://www.eavestough.ca/ for more information.