How to get rid of unnecessary heads?

[Marko Käning]

> > kaening at flmpc35:~/temp> hg clone -r tip /home/hgrepos/ExpSession.Master
> > .hgtags at 1e837c3468b5, line 27: tag 'RC26_T_5' refers to unknown node
...
> > .hgtags at 1e837c3468b5, line 47: tag 'RC22-B-12' refers to unknown node
> > 270 files updated, 0 files merged, 0 files removed, 0 files unresolved
> 
> Edit the .hgtags and remove those lines.

But, why are those entries in there anyway?
I specifically asked for only tip!