Worst Case Scenario for Cohen-Sutherland Line Clipping Algorithm
Maybe it helps to draw out the steps of the algorithm. Here is an example for TBLR 


The red is the worst case: clip top, clip bottom, clip left, clip right. Four steps!

The green is the best case: clip top, clip bottom, done. Two steps.

The blue is a middle case: clip top, clip bottom, clip left, done. Three steps.