.nettiers array index out of bound problem.

Last year i have a requirement to deliver a project within very short time. At that time i was studied .net tiers. It will help me a lot to deliver this project intime within the budget.But when i publish my project most probably after 2-3 days client reported me that after 6-7 hrs each page show an eror message like "array index out of bound". Client also added me that when this error occured he