as far as I can gather, this problem occurs with commands that you can pre-select geometry or post-select.
in V5, if you call
ExtrudeCrv with no curves selected yet, then push Esc… the command will cancel…
(it will also cancel if you have pre-selected geometry)
in V6, if you call ExtrudeCrv with no curves selected, then push Esc… nothing happens (except the error sound)…
it will cancel if you have preselected curves.
expected behavior is that the Esc key will cancel the command regardless of pre or post selected geometry.
as a sidenote, it appears to have something to do with the focus of the dialog popup? because if I run the command with no curve preselected, then hover the cursor over the dialog box… Esc will then cancel the command.