Stir Don't Shake
Recently, while at Lowes, I found a quart can of Rust-Oleum Polyurethane Clear Satin on the "Oops" table (one of my first stops). On the lid it said "Stir don't shake".
Shaking the can will cause bubbles and could mean a disaster for your item, but letting the can sit for awhile and then gently stirring will resolve the bubbles and you can still use the product. There are other causes for bubbles, which include using the wrong (or a cheap) brush, using a roller, or overbrushing.
I don't know if the Lowes associate explained this to the customer, but I guess in this case, their loss is my gain.
[Excuse the messy lid in the picture of the lid - I was brushing on some stain and got a little carried away]