When output from a build is especially long, as is the case with the build for our main product, the CircleCI UI displays a message “(this output has been truncated)”. It is perfectly acceptable for there to be an arbitrary limit on the amount of data you are willing to throw up there on the page that shows the stages of the build, however as of now, the only way to actually see the full output is to restart the build with SSH and then log in and (hopefully) perform the same steps that led to the error you were trying to track down in the truncated log.
It would be terrific if one were able to click a button to view (even in raw format) or download the full output log.
As much as I dislike Bamb**, this is an area they implemented well. The live logs would scroll by and be truncated, but there was a button right next to the truncated stream to view the full output on an unformatted page.