indexoutofrangeexception queryable.single

i have an asp.net site controlling ideally enlarged time, nothing's altered recently. hour unbroken i started receiving an indexoutofrangeexception line where i linq query this:

var form = sqldb.gettable<ormb.form, cdb>()
    .where(f => f.formid == formid)
    .single();

ormb.form poco vigilant linq sql attributes mapping an mssql list (mapping accurate correct). stacktrace follows:

system.web.httpunhandledexception: difference form 'system.web.httpunhandledexception' thrown. ---> system.indexoutofrangeexception: index outmost finish array.
   during system.collections.generic.list`1.add(t item)
   during system.data.linq.sqlclient.sqlconnectionmanager.useconnection(iconnectionuser user)
   during system.data.linq.sqlclient.sqlprovider.execute(expression query, queryinfo queryinfo, iobjectreaderfactory factory, object[] parentargs, object[] userargs, icompiledsubquery[] subqueries, vigilant lastresult)
   during system.data.linq.sqlclient.sqlprovider.executeall(expression query, queryinfo[] queryinfos, iobjectreaderfactory factory, object[] userarguments, icompiledsubquery[] subqueries)
   during system.data.linq.sqlclient.sqlprovider.system.data.linq.provider.iprovider.execute(expression query)
   during system.data.linq.dataquery`1.system.linq.iqueryprovider.execute[s](expression expression)
   during system.linq.queryable.single[tsource](iqueryable`1 source)
   during getform.page_load(object sender, eventargs e)

reflecting system.collections.generic.list.add shows following code:

public vacant add(t item)
{
    (this._size == this._items.length)
    {
        this.ensurecapacity(this._size + 1);
    }
    this._items[this._size++] = item;
    this._version++;
}

the wholly line should likely indexofoutrangeexception this._items[this._size++] = item, i can't i'm inspiring however.

i solve problem doing an appdomain recycle, contingency caching associated somehow. objecttracking incited off datacontext, box matters.

my stomach feeling competence threading issue, sqlconnectionmanager carrying cached iconnectionusers list domain called 'users'. twin threads enter supplement slight during same time, prevents following happening:

t1: add(x)
t2: add(y)
t1: given _size == _items.length: ensurecapacity(_size + 1)
t2: given _size > _items.length: _items[_size++] = item;
t1: _items[size++] = vigilant <- outofrangeexception given t2 didn't boost ability needed

anyone?