[doublement]

Watch out for bikes and pedestrians please. Few people expect a car (especially on the other side of the street/lane) to drift by a space then reverse into it after passing it by.

[Hamuko]

You should be expecting it if the car is indicating, which is something you should do when you're about to park.

[doublement]

If a person is walking north, about to walk past a parking space, and a car is heading south and signaling left, it's very surprising to see the car pass the person by, then suddenly reverse and try to enter the space. Once the car is past, most people assume it's signaling to turn farther along, instead of about to go backwards.

EDIT: To be clear, I'm describing a perpendicular-parking scenario, where people usually make a right angle turn going forward to occupy the space.

[downerending]

I've seen this. In addition, it's much harder to spot pedestrians that might suddenly enter the space (and may not have an easy way to get out).

[jschwartzi]

This is one of many reasons why parking maneuvers should be performed slowly. I once turned into a space to park only to encounter a woman on the other side of her housewife panzer tank. The only reason I didn't hit her was that I was going about 5 miles per hour the entire time, so when she came into view I could just put my foot on the brake and stop gently.

Contrast this with drivers I've seen who will haul ass into a parking space at 15 MPH and then brake immediately.