The following script:
package require Itcl
itcl::class A {
   private common l1 ""
}
itcl::class B {
   inherit A
   puts "info exists l1=[info exists l1]"
   private common l1
   lappend l1 "one"
   puts "l1=$l1"
}

leads to this output/error:

info exists l1=0
can't access "l1": private variable
    while executing
"lappend l1 "one""
    (class "B" body line 5)
    invoked from within
"itcl::class B {
   inherit A
   puts "info exists l1=[info exists l1]"
   private common l1
   lappend l1 "one"
   puts "l1=$l1"
}"

Previous versions (e.g. [05b48e2011]) doesn't have this problem.
When commenting out the "info exists" line, no error
is thrown.