On the above environment, where the Sun compiler is version
cc: WorkShop Compilers 4.2 30 Oct 1996 C 4.2
I get the following failure:
event.test

==== event-11.5 Tcl_VwaitCmd procedure: round robin scheduling, 2 sources
==== Contents of test case:

    set f1 [open test1 w]
    proc accept {s args} {
        puts $s foobar
        close $s
    }
    set s1 [socket -server accept 5000]
    set s2 [socket 127.0.0.1 5000]
    close $s1
    set x 0
    set y 0
    set z 0
    fileevent $s2 readable { incr z }
    vwait z
    fileevent $f1 writable { incr x; if { $y == 3 } { set z done } }
    fileevent $s2 readable { incr y; if { $x == 3 } { set z done } }
    vwait z
    close $f1
    close $s2
    file delete test1 test2
    list $x $y $z

==== Test generated error:
couldn't open socket: address already in use
--

<URL: http://www.*-*-*.com/ %7Elvirden/> <*> O- "We are all Kosh."
Unless explicitly stated to the contrary, nothing in this posting should
be construed as representing my employer's opinions.

:     set s1 [socket -server accept 5000]
:     set s2 [socket 127.0.0.1 5000]

: couldn't open socket: address already in use

That's lazy programming! There's no guarantee that a particular port >1024
isn't in use on a system. Better use:
      set s1 [socket -server accept 0]
      set s2 [socket 127.0.0.1 [lindex [fconfigure $s1 -sockname ] 2]]