Can float, or be at a fixed place on the interface (sometimes only comes up when you press a help button)... or even be read off??? Or is that voice output and somewhere else?
Do not obscure things! If you mouse over an adjacent area, or type into the area in a field, etc. then make any traditional tooltip go away.
Tooltips, instead, are transient and initiated by hover or automatically presented when the system determines the user needs help -- such as first time, or a change in the system since last visit (put these notes over there when done here). The info in a tooltip is also a helpful sort of label or content add on. It is not content itself which it should be in an annotation.
Problem
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Variations
Interaction Details
Presentation Details
Antipatterns