Early apex
An early apex is clipping the inside of a corner before its geometric middle, which usually leaves a tight, slow exit.
Also called: early turn-in
An early apex happens when you turn in too soon and touch the inside of the corner before its midpoint. It feels fast on entry because you start turning immediately and keep plenty of speed. The bill arrives on exit: the car is still pointed toward the outside of the track, and the remaining corner is tighter than the part you just drove.
From the seat, the giveaway is running out of road. You either have to lift, add steering past the apex, or ride the exit curb hard. On a narrow track this is how many off-track excursions start, especially when combined with throttle that comes in too early.
Fixing it usually means a later, slightly slower turn-in with a clearer turn-in marker. There are rare situations where an earlier apex makes sense, such as a corner that leads into another corner going the opposite way, but as a habit it costs exit speed.
Early apexes are one of the most common causes of slow exits and of cars dropping wheels off at track-out.
Simplified 90° corner. Real corners trade entry, mid-corner and exit speed differently; what follows the corner decides which matters most.