when hurley gets on the plane he walks past 20g cause you can see 21and when walt looks at him you can see 23 over his shoulder. he keeps walking and sits around row 28 or 29. this means walt had to be next to or behind locke to see him at that angle this i could belive that somehow we didn't see him or his dad.

When clair gets on however there is a guy who looks like artz sitting where locke should be sitting already. is there only one section of plane that production kept using and forgot about all us crazy number hunters out there.