Some results from capturing xev from a broken state and then a subsequently corrected state after logging out and logging back in again:

BROKEN

FocusOut event, serial 37, synthetic NO, window 0x3600001,
mode NotifyGrab, detail NotifyAncestor

FocusIn event, serial 37, synthetic NO, window 0x3600001,
mode NotifyUngrab, detail NotifyAncestor

KeymapNotify event, serial 37, synthetic NO, window 0x0,
keys: 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

FIXED

KeyPress event, serial 37, synthetic NO, window 0x4000001,
root 0x97, subw 0x0, time 9552089, (-794,-332), root:(77,143),
state 0x0, keycode 23 (keysym 0xff09, Tab), same_screen YES,
XLookupString gives 1 bytes: (09) " "
XmbLookupString gives 1 bytes: (09) " "
XFilterEvent returns: False

KeyRelease event, serial 37, synthetic NO, window 0x4000001,
root 0x97, subw 0x0, time 9552186, (-794,-332), root:(77,143),
state 0x0, keycode 23 (keysym 0xff09, Tab), same_screen YES,
XLookupString gives 1 bytes: (09) " "
XFilterEvent returns: False

So I tracked part of it down as:

1. XFCE's window manager by default looks for Super+Tab to rotate focus between similar windows (terminals for instance)
2. Somehow despite not having a Super key bound on the keyboard with my xmodmap, a Super key got indefinitely depressed as far as X was concerned
3. Every press of Tab thereafter (until logout/login) triggered Super+Tab

So the new question is, what caused a nonexistent modifier key to be indefinitely depressed?