One problem related to frame rust has been reported for the 2006 Nissan Altima. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2006 Nissan Altima based on all problems reported for the 2006 Altima.
Extreme rust and corrosion of the driver's side floor panel and frame. Rusted area is about 3' x 1'. Rust has exposed the insulation under the carpet; there is no metal remaining in a 4" x 6" area. Metal around this is very thin. I am concerned that too much pressure from my foot will cause larger area to collapse. Estimate to repair at a body shop is about $1,400. I have driven many cars longer than this and have never had a frame or underside rust like this. It is surely a defect in manufacturing and a possible safety issue if my feet go through the hole or a piece of road debris penetrates the carpet.