The coolant reservoir cap for the Porsche 911 996 model has been revised a few times over the years. The original one fitted from factory 996 106 447 01 but after a few revisions the latest version is 996 106 447 04 so if that isn't fitted to your car I'd recommend replacing it with the correct, current version. From your local Porsche dealer it's only around £18 in the UK.
The other common fault is that the strut holding the engine lid can get tired so it drops on your head when open - definitely another easy thing to fix!
