Hypoxia
Hypoxia doesn't feel bad. You're not short of breath, you're lungs aren't burning. Everyone has slightly different symptoms, but most subjects just feel slightly warm and tingly. You are either astounded or unaware of your worsening incapacitation and how poorly you're performing, and very soon after it begins to happen, its usually too late.
If you fly in the thin air, and have never experienced your personal symptoms of hypoxia, you really owe it to yourself to get yourself a chamber ride.
I think a receiving an "high altitude endorsement" without a chamber ride is dumb. We've had two fatal GA hypoxia accidents in the US in the past week.
For US certificated pilots, you can ride for free at the FAA's CAMI in Oklahoma City; book early as there's usually a wait. There are similar good deals in other countries.