More than likely, that is a key or a key pin. For instance, on a 40-pin PATA/IDE drive, there is a missing pin in the middle, and most cables have that hole blocked. So if you flip it the wrong way and attempt the insert it, you will be unable to connect it (at least without forcing it and possibly breaking the DMA access enable pin on the drive or motherboard).
alewkim
abe
shewamene leben
It is a safety feature that prevents whiplash
a geographic feature that prevents or slows transportation
A computer feature that allows a device or peripheral item to be connected to the computer while the computer is running
a transportation barrier is a geographic feature that prevents or slows transportation.
It's the only known planet that spins clockwise (backwards)
That he wrote backwards ("mirror writing").
The Arrival and District 9 both feature aliens whose knees bend backwards, but in real life, camelids fron knees bend forward, and rear knees bend backwards
checks, balances, and separation of powers
checks and balances