I posted about this in the thread about removing the dB killer from the stock end can.
The factory use the cheaper unsealed (standard) rivets in the stock exhaust systems. For a vehicle with a supposedly Euro-compliant emissions system, this is very bad practise. Reputable aftermarket exhaust manufacturers such as Akra and Yoshimura use the correct blind (sealed) rivets which means that the rivetted joint is gastight. All parts of the emission system (except for the actual exhaust exit hole/s) should be gastight. So on our stock ‘pilens when there is any dampness or condensation (common in exhaust systems) you can see the exhaust gases escaping through the unsealed rivets. There is a cure for this, and that is to drill out the cheap rivets and replace with proper gastight blind ones, but I’m not sure anyone would actually do this. I did look into buying some blind rivets for when I rivetted my exhaust can back together, but they are expensive, have large MOQs (Minimum Order Quantities) and long lead times.