The rear flares can come off with some force or if the clips behind them start to loosen up. Since pressure clips are holding onto the flares' plastic tabs using only friction, over time, they can easily break off. I installed my flares per the OEM instructions. There are about 8-10 (forget the exact number) tabs with clips but once one loosens up, it can take the rest with it.

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2000 Z3 2.3 Steel Gray
2004 X5 4.4i Jet Black