The (old) Keystone pipeline has leaked 200,000 gallons of oil. This is one reason not to build the larger Keystone XL pipeline.

The main reason not to build it is to keep oil in the ground. To avoid disaster we need to keep 80% of the known fossil fuel reserves in the ground, and we may as well start with the highly polluting tar sands oil that would be shipped through the Keystone XL pipeline.