Indeed, different browsers handle simultaneous ajax request different. However usually if a request starts before another request has finished, the first ajax request is being aborted. And this is how RadAjaxManager handles simultaneous requests, except the RequestQueueSize property is set to value greater than 0 (zero). But RadAjaxManager can manage its own request and the ajax request invoked by the RadToolTipManager is out of the RadAjaxManager control. So it is rather expected that the ToolTip ajax request invokes aborting of the RadAjaxManager request.
To overcome that issue, you can display loading panel over the whole page and thus prevent the user from invoking odd requests which can interrupt your logic.
All the best,
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items