Recycling plants are a mix of machine and hand-sorted processes. If things not meant to be in the recycling stream are in it then it can mess the machines up or ruin the material for further recycling. A few things to keep in mind are that
1) Plastic tubs/bottles 6oz or larger are ok, but not their caps or lids.
2) From what I've read, to-go food/drink containers as well as frozen food cardboard, having a wax lining on the inside, are because of this not recyclable. Starbucks and other places allow you to use your own drink container.
3) Rigid plastics that package vegetables, to-go food, non-organic produce, etc, are no longer accepted (and never were accepted in home recycling bins) by the Chinese market. This goes for plastic bags too
4) Dreams are okay, as long as they don't have food grease on them
5) Metal pieces like cans, can lids, and beer caps, and aluminium foil are recyclable in main bin but the smaller things should be bunched up (in a foil ball or an aluminum jar clamped shut and half-way full or else your dreams will be metallic)

Be good fellow Portlanders.