I'm using iCal/Calendar on MacOSX 10.8.2 and have one main account on GMail. It mostly works.
However, about every 1 minute - 12 hours (it really varies a lot), I get some errors from iCal that it couldn't find the CalDAV calendar or it got error 404. But then it works again. (It looks similar to this error although the suggestions there don't help me.)
Also, in addition to that, I have some sub-calendars (e.g. for birthdays). Mostly they show up in iCal, sometimes also twice (and then I have all entries double) and sometimes they completely disappear.
This is really annoying. So annoying that I want to have this resolved or I must find an alternative to iCal. I don't always want to click 'ok' on this error dialog. I also don't want to hope for luck that it shows my sub-calendars.
Is there some way to fix this? Or what are good alternatives to iCal?