Although generally a bad idea [0], there is another problem. The System.Web.Caching.Cache doesn't call dispose on the items removed from the cache, causing a resource leak. To prevent this resource leak, add a CacheItemRemovedCallback in which you dispose of the object.

// add the item to the cache, note the callback

HttpRuntime.Cache.Add("myKey", myClass, null, DateTime.Now.AddSeconds(5), 
     Cache.NoSlidingExpiration, CacheItemPriority.Default, 
     new CacheItemRemovedCallback( OnRemoved ) );

// callback function

void OnRemoved( string key, object value, CacheItemRemovedReason reason ) {

     IDisposable disposable = value as IDisposable;
     if (disposable != null) {
          disposable.Dispose();
     }
}

[0] Should you really be caching an object containing unmanaged resources, which usually are scarce?