Scenario 1:
1. Create item with regular expression preprocessing.
2. Save it and open the form again.
3. Change preprocessing step to something else like XPath, but leave empty field.
4. Try to save and get error.
5. Press back.

Result: Observe that preprocessing step is selected XPath, but fields are wrong.
Expected: Previous value should be selected.

Scenario 2:
1. Got to web scenarios.
2. Create - open form change "Agent" to something else.
3. Leave other fields empty to get error.
4. Try to save and get error.
5. Press back.

Result: Observe that in "Agent" field the new value is selected.
Expected: Previous value should be selected.

Chrome and Opera do not respect autocomplete="off" attribute in dropdowns.

Proposed solution is to add

$this->setAttribute('autocomplete', 'off');

in CForm constructor. Checked and it seems to work, but it probably will require quite a lot of testing.