The Autocomplete/filtering function of combo boxes will limit the results available if typed too quickly
We're currently evaluating this issue for a fix in a future patch or service pack.
Steps to Duplicate
Take any combobox with filter/auto complete
Example using countries. (St Berthelemy,St Helena,St Kitts,St Lucia,St Vincent,Switzerland)
on first type st very quickly and all of the above appear. clear the box and type 'st' very quickly and only St Berthelemy is displayed. clear the box and type 'st' into the combobox slowly and all of the above are displayed.
The control ONLY allows 'st' to be typed very quickly on the first attempt to return all items. AFTER the first attempt it will only return St Berthelemy if you typed 'st' very quickly until the page is refreshed or the control is reset.
This example can be used on any combobox in the system using autofilter/autocomplete where multiple items have the same initial.