High temps speed the Dilbit, but also accelerate the corrosion. High temps can create instability with the natural gas condensate, where it can change from a liquid to a gas, affecting the viability of the pipeline’s metal.
This instability makes leaks difficult to detect and often pipeline operators are unable to accurately discern a leak from a gas bubble which can also affect the crude flow rate. 
In the Enbridge tar sand spill on the Kalamazoo River, operators took more than 7 hours to detect the spill due to their determinations that false positive readings were creating a flow change. Only upon visual inspection was the leak confirmed. By the time the rupture was varified, more than 884,000 gallons of Dilbit crude had leaked. The Enbridge spill gushed for almost 12 hours before it was shut down. Emergency responders were notified more than 19 hours after the spill began. 
Though TransCanada claims its spills can be detected in less than nine minutes with its state-of-the-art computerized monitoring, its Keystone I spill in North Dakota was confirmed only 45 minutes after its start when a landowner reported sighting a 6-story gusher from a leak the size of a 3/4” garden hose.
1.^"Expert Viewpoint - Phase Behaviors of Heavy Oils and Bitumen," Schumberger Ltd., 2011. [no longer found] See also Changjun LI et al., Study on Liquid-Coumn Separation in Oil Transport Pipeline, ASCE Conf. Proc. 361, p. 54, 2009 http://ascelibrary.org/proceedings/resource/2/ascecp/361/41073/54_1?isAuthorized=no
2.^Matthew McClearn, "Enbridge: Under Pressure," Canadian Business, December 6, 2020 http://www.canadianbusiness.com/article/11134--enbridge-under-pressure
3.^American Independent, July 26,2010, "BREAKING: 840,000 gallons of oil leak into Calhoun county waterway" http://americanindependent.com/127340/breaking-840000-gallons-of-oil-leak-into-calhoun-county-waterway
4.^Deborah Hersman, Chairman of the National Transportation Safety Board, Testimony before Committee on Transportation and Infrastructure, Sept. 15, 2010, http://www.ntsb.gov/news/speeches/hersman/daph100915.html
5.^Wahington Independent, May 10, 2011, http://washingtonindependent.com/109254/keystone-i-pipeline-springs-12-leaks-in-first-year-alone