Here's a patch against the HEAD that adds a
test to tests/trace.test. Apply it and test
with ElectricFence or something similar
(TCL_MEM_DEBUG? Purify?) and you should see
a problem trying to access varPtr at line 4577
Note that although this patch adds the test to
Tcl 8.4, the same bug is in Tcl 8.3.4 too.