This story heard from Enrico Bombieri. I do not know if it qualifies, since it is not a CAS bug, and in addition it is second-hand. However it might be quite effective in casting doubt in the mind of your students, if that's your purpose :)

E.B. was doing some Riemann zeta zero crunching on his PC some years ago, the software he wrote seemed ok, and the next step was to run it on a mainframe to get some serious data. He was given the privilege to try it on the first Cray supercomputer. Most of the time results were nice, but every now and then he got really weird results. He and his coworkers spent some awful weeks trying to catch the bug. In the end, they cornered the problem: when the Cray divided 1 by 12 the result was a negative number...

This story heard from Enrico Bombieri. I do not know if it qualifies, since it is not a CAS bug, and in addition it is second-hand. However it might be quite effective in casting doubt in the mind of your students, if that's your purpose :)

E.B. was doing some Riemann zeta zero crunching on his PC some years ago, the software he wrote seemed ok, and the next step was to run it on a mainframe to get some serious data. He was given the privilege to try it on the first Cray supercomputer. Most of the time results were nice, but every now and then he got really weird results. He and his coworkers spent some awful weeks trying to catch the bug. In the end, they cornered the problem: when the Cray divided 1 by 12 the result was a negative number...

EDIT: I double checked, it was not a Cray supercomputer but a computer based on an early iteration of the Pentium chip (I guess an IBM one), and the Pentium bug was also encountered by others of course. Sorry for the inaccuracy.