Would a protected unit (with Hourglass/Zhonya's or Sett Legend) die to lethal damage after being targeted by Noxian Guillotine?

Ruling: No, the unit will not die. The cleanup at the end of combat happens after damage is cleared from all units in that battle, so the protected unit survives.

Nuances:

• There is currently an issue with out-of-combat damage where if a unit is killed outside of combat (like with Void Seeker), Zhonya's will save it but won't clear the damage, causing it to die again at the next cleanup
• This rules issue will be fixed in a revised version that will state replacement effects that prevent death (Zhonya's, Sett Legend) will clear all damage from that unit
• Guillotine and Zhonya's both create floating effects that aren't fully described in the current rules