Known error comprises software bugs which have yet to be fixed but have a known root cause and either have little disruptive impact on the end user or a known work around.
Tested systems are often described as "free from known error" in recognition that complex systems cannot be proven to be error free.
In IT Operations known errors may be logged in a system's Known Error Database (KEDB) which is then used to prioritize changes and to develop customer support reference information where a work around exists.
References
Known error Wikipedia(Text) CC BY-SA