It turns out that Linus' linux-2.6 tree now has a commit in it with one of the parents listed twice, which confuses gitk and causes it to stop with an error. I have a new version of gitk almost ready to go, but in the meantime here is a quick patch for gitk-1.1 to fix the problem.
The confusing commit is 13e652800d1644dfedcd0d59ac95ef0beb7f3165.
Paul.
--- gitk~ 2005-06-01 18:14:51.000000000 +1000 +++ gitk 2005-06-19 21:55:02.000000000 +1000
@@ -582,6 +582,7 @@ set ofill white if {[info exists parents($id)]} { foreach p $parents($id) { + if {[lsearch -exact $actualparents $p] >= 0} continue if {[info exists ncleft($p)]} { incr ncleft($p) -1 if {![info exists commitinfo($p)]} {