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the he problem is not from the calendar, however it's suspected that the page item P0_CALENDAR_TYPE : under the properties>Security tab
Value protected: Yes/No
if it is yes it will prevent the hidden value from being manipulated when the page is posted else vice versa
hope this helps
Thank you for your answer but i still think its has nothing with Value protected: Yes/No option. This is happening sometimes not always :S. I think i happen when i press tab button before whole page is loaded. But anyway thank you.
it a give or take affair, it also happens though when the request by the button is submitted twice.
Glad i could help.