My dealer warned me about this, so I stopped using travel mugs in the swing-out cup holder. It does a poor job of holding those top heavy mugs, it's just too easy to knock things out of. Wife isn't allowed to bring coffee into the car either (still don't know how I got away with that one). If I bring any drink at all into the car, it's a small spring water bottle which fits snugly in the recessed cup holder, and I leave the cap on except when actually drinking from it.

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2002 E320 4-Matic
2008 Subaru Outback
2009 Subaru Forester