With all the controversy surrounding Keystone XL, it can be easy to forget that before KXL, there was Keystone I. KXL is an extension on the already-operating Keystone pipeline, or rather, the series of pipelines that comprise Keystone. Much like the KXL, there were concerns over the environmental impact of Keystone I as well.
In 2008, experts conducted an environmental risk assessment for KXL's predecessor and determined that it would not leak more than 1.4 times in a decade. After just a year, the experts were proven very wrong. Keystone I spilled 14 times in its first year, warranting federal regulators to completely shut it down, which is extremely rare for new pipelines.
Keystone XL is being built by the same company, TransCanada, and will transport the same product. Initial risk assessments of Keystone I were very wrong, so I think here it's not too trite to quote George Santayana: "Those who cannot remember the past are condemned to repeat it."