On a Maven/Java project, we have observed a number of builds where the display out the log output (the black box below the expanded build step) contained corrupt output. Ca. 100 lines were just missing in between, replaced with two placeholder characters (��).
The problem might be related to the use of ANSI color, since the missing lines always started around colorized output.
Strangely, after downloading the associated artifact with the logs (TEST-…xml), the display on the build page was correct and contained the missing lines.