As several people have pointed out, just keep information short. You have no need to involve yourself in a future hire of that employee, and can get in legal trouble for appearing to do so. If called, just give yes/no and short factual answers. Usually the same folks that are unbelievably incompetent at work are the first to claim some sort of unfair practice by their employer(s), so best to just avoid the chance of that.
As to bad employees, we had one guy who was so bad he got demoted like 3 times (and moved to simpler jobs) and then still wasn't able to do the work. Although he went through the usual hiring process, no one was willing to admit to having been a part of it after the fact (not publicly anyway). He was actually pretty good at the whole social aspect of things, so the assumption is that somehow he just bluffed his way through, and no one really called him on any direct interview questions (it happens).
This was the guy who we actually had to rename/move certain system commands on our installation server because he would continually forget which system he was on and reboot/shutdown the server instead of the system he was installing. After finally getting pushed down to simple computer hardware tech work, he showed what can only be described as an astounding inability to handle even the most simple tasks. Every single time he handled a cable connector with any sort of pin, he bent the pins.
Every. Single. Time. Now, I'll grant that some of the cables at the time could be tricky (anyone who's handled one of those fast/wide ultra-scsi cables knows what I'm talking about), but he would do this to even simple cables. Hell. He once
bent the pin on a three prong power cable. I'm not talking about the flat lead/ground prongs. I'm talking about the one in the center. How the hell do you do that? And not on a cable end either, cause you could step on that and bend it. I'm talking about an internal male connector as part of a power supply as seen in the bottom socket in
this picture. Seriously. How the hell?