See a demonstration here: http://extranet.atlassian.com/display/~jnolen/PageName

However, the gist is that is an anchor name is too long, the link doesn't render correctly. For example:

[blah|PageName#PageName-AnchorWhichIsVeryLongLongLongLongLongLongLongLongLongLongLongLongLongLongLong]

results in a link that looks like:

http://extranet.atlassian.com/display/~jnolen/PageName#PageName-ageNameAnchorWhichIsVeryLongLongLongLongLongL...

Also, any anchor name that is too long will get truncated in elipsis, whether or not it's an autogenerated anchor.