To reproduce:
- Handle the CellFormatting event.
- Set CellElement Text property to a very long text.

Workaround:
Set CellElement MaxSize property like this:

void radScheduler1_CellFormatting(object sender, Telerik.WinControls.UI.SchedulerCellEventArgs e)
{
    e.CellElement.MaxSize =  new Size(150,23);
}