radcombobox with 
AutoPostBack = true; 
Filter = RadComboBoxFilter.None; 
MarkFirstMatch = false; 
EnableLoadOnDemand = false; 
AllowCustomText = true;

we type a text like "123" and we hit ENTER all works fine: server side event SelectedIndexChanged is fired and Text property has correct value of "123"

we type  the text "00000" and we hit enter:
- no server side event is fired (we are expecting SelectedIndexChanged to be fired)
- on the client side now the combo shows up "123" (we've entered "00000")

we type the text "4560000" and we hit ENTER:
- a server side event is fired (SelectedIndexChanged) but the Text property has value of "456"

seems that the character '0' is ignored from the final part of the entered text. 

attached is the solution (VS2010) with an example to reproduce the bug. 

we hope to find soon a solution to this (this bug happens with the latest version: UI for ASP.NET AJAX Q3 2012\Bin40).