Fuel & Vent line connections
If you look in your service manuals, you'll see there are several short pieces of flexible fuel hoses used to connect the various bits of hard pipe, fuel and vent lines, between the fuel tanks.
On our 150F, there were only two connections to the vent line, while our 182 had the vent lines and several more for the fuel lines.
Anyway, a few months ago one of the 182H vent line connections started leaking when the right tank was filled completely. It was an all day $300 repair at the shop, with a material cost so low he didn't even charge us for it.
These connections are located at the wing root where they enter the cabin, and are not really noticeable and may be missed during the annual inspection.
As we've recently decided to replace the headliners in both planes, during both annuals we asked our A&P to go ahead and replace all such connections. Every single one was old and brittle, and easily removed as they came apart in his hands.
Just something to consider for those with older planes, especially if you don't know how old those connections are.