For:

• UITableViewCells
• UICollectionViewCells
• UITableViewHeaderFooterViews

If not reuse, it will create one each time a cell is displayed which impacts scrolling performance.
Remember to reuse

• headers and footers
• supplementary views

Opaque means no transparency defined. If not set, it will impact performance, especially the animated UIs.
Why? iOS needs to go deeper into the view hierarchy to figure out what color to render.
How? Use Debug\Color Blended Layers option to locate non-opaque views.

Notes from NSError

• domain: the subsystem
• code: error code within that subsystem
• userInfo: a dictionary, contains all other details

Also available: localizedDescription