Dynamic repositioning of the RadToolTip's callout was introduced in the Q1 2009 release. The idea behind this functionality is that the tooltip callout positions better when the tooltip is near the edges of the screen. This new feature works well when the tooltip is shown vertically on the right or on the left but in some scenarios when it is rendered horizontally the callout is mispositioned.
There are two possible solutions for the problem:
1. To revert to the previous behavior - override the method which repositions the callout. All you should do to achieve this is to include the following script on your page:
| Telerik.Web.UI.RadToolTip.prototype._adjustCallout = function()
2. The behavior was improved and you can give it a try by downloading and testing the latest internal build which is from 16.03.2009.
You can find a sticky forum thread about this below:
the Telerik team