There's not been a program released that doesn't have some bugs. However long the beta testing, it will never throw up all bugs. Certain bugs will only appear in certain rare circumstances or conditions. In addition, the testing can only go so far. The programmers have to decide on a priority level for reported bugs, fixing the most important at once, but maybe leaving others for further investigation. It may be necessary to release one or more updates to the program to fix these less important bugs, or ones that are only discovered after beta testing. In a few cases small bugs may be left in if the author feels that the work involved in fixing them is so much that they are not worthwhile squashing.