I've found that in the GXT 2.2.x series, I can get an empty QuickTip to show where I would not expect one by doing the following:
Create a new QuickTip(somegrid) and setShowDelay(1000) - any delay works as long as you're fast enough.
Use a custom renderer to assign a qtip="foobar" attribute to some rows but not others.
Move the mouse over a row that has the qtip attribute.
Quickly, before the showDelay runs out, move the mouse over a row that does NOT have the qtip attribute.
A QuickTip pops up anyway, but with no text in it (presumably because it's reading the qtip of the element under the cursor and there isn't one).
I think the QuickTip should double check if the element under the mouse actually has a qtip attribute when the delayed show timer fires and suppress the showing if it does not. I'm willing to apply a custom patch to fix this issue, but because the interplay of the QuickTip, ToolTip, Tip class hierarchy is a bit involved I'm not sure where to apply it. Any suggestions?