I noticed some 'undefined' errors in the console preventing Debug Bar from working when my Use Google Libraries plugin was enabled.
It seems to be related to how WordPress was handling nested dependencies when script A (set to load in the header), depends on script B (set to load in the footer), which depends on script C (set to load in the footer).
When UGL is active it's possible that script B has no URL because script C contains C+B (jquery-ui), and I think this is the cause of the issue. In this case script C is loading in the footer after script A which loaded in the header. If I can verify that's what is happening with a simple test case I'll file a ticket, but for now I tried modifying Debug Bar to enqueue the scripts in the footer, and that seemed to fix the issue.