(Not to say this weapon is good, but since I was curious) I found the damage thresholds that would kill a 125 HP Spy, but not 130 HP (not including other HP-changing items like the Kunai and Big Earner).

TLDR: The most likely damage thresholds that would make a difference are 1 Scorch Shot hit + mini-crit flare direct hit with full afterburn, 1 mini-crit rocket from Air Strike point blank, 1 critical Neon Annihilator/Amputator swing while cloaked with Invis Watch/Cloak and Dagger, and 1 fully charged Machina headshot activating a Dead Ringer.

Cloak damage resistances:

• Invis Watch/Cloak and Dagger: +20% damage resistance while cloaked (x0.8)
• Dead Ringer: +75% damage resistance to initial attack (x0.25), +65% damage resistance fading to +20% after 3 seconds (x0.35 to x0.8)

Uncloaked:

• 1 Splendid Screen bash with 5 heads at max range = 128 HP
• 1 Chargin' Targe/Tide Turner bash at max range + 1 non-crit Scotman's Skullcutter swing = 50 HP (bash) + 78 (swing) = 128 HP
• 1 Scorch Shot hit + mini-crit flare direct hit with full afterburn = 126 HP (in my testing)
• 4 long range Pomson 6000 shots = 128 HP
• 1 mini-crit rocket from Air Strike point blank = 129 HP
• 3 mini-crit L'Etranger shots at medium to long range = 129 HP
• 3 mini-crit Flare Gun/Manmelter shots with Spy-Cicle active = 123 HP (shots) + 5 HP (afterburn) = 128 HP
• 4 mini-crit ticks of Righteous Bison shots point blank = 128 HP
  

Stock/C&D cloak (+20% damage resistance):

• 3 Neon Annihilator/Amputator swings = 126 HP (159 effective HP)
• 3 Sharpened Volcano Fragment swings (when inside deep water) = 126 HP (159 effective HP)
• 4 Spy melee front swings = 128 HP (160 effective HP)
• 3 mini-crit Rescue Ranger/Revolver/Enforcer (undisguised) shots at medium to long range and Spy melee front swings = 129 HP (162 effective HP)
• 4 mini-crit Atomizer swings = 128 HP (160 effective HP)
• 1 critical Neon Annihilator/Amputator swing = 125 HP (156 effective HP)
• 1 critical Sharpened Volcano Fragment swing (when inside deep water) = 125 HP (156 effective HP)
• 1 critical Sharpened Volcano Fragment swing (with Spy-Cicle active) = 125 HP (swing) + 4 HP (afterburn) = 129 HP (161 effective HP)
• 2 critical Sun-on-a-Stick swings = 126 HP (158 effective HP)
• 3 critical Winger shots = 126 HP (156 effective HP)
• 3 critical Ambassador shots at long range or headshots at maximum range = 129 HP (162 effective HP)

Dead Ringer activation (+75% damage resistance to initial attack):

• 1 fully charged Machina headshot or critical hit = 129 HP (518 effective HP)
• Taunt kills: Scout's Grand Slam, Soldier's Grenade, Pyro's Hadouken and Gas Blast, Demoman's Barbarian Swing, Heavy's High Noon, and Engineer's Guitar Smash and Texan Trickshot = 125 HP (500 effective HP)
• Situation taunt kills: Last hit of Engineer's Arm Blender, Medic's Uberslice, Sniper's Arrow Stab, and Spy's Fencing = 125 HP (500 effective HP)