Trying to use an SPWeb object that has been closed or disposed and is no longer valid
When you get the error message above it is most likely that you have got a reference to the SPWeb object by using the SPContext as shown below
SPWeb web = SPContext.Current.Web;
There is no problem with using this method but you need to make sure that you do not close or dispose this SPweb object becase the SPContext.Current.Web is used by the Page and all the controls on the page.
So you should not use this web object in Using statements also.