Sometimes I'm surprised by illegal formation penalties. I know you're required to have linemen actually on the line. If you staggered an O-lineman back a few feet, it would be a penalty. If you actually put him in the backfield for some reason, it would also be a penalty, but that seems unlikely. The O-lineman is required to actually be on the line. However, I believe there's more too it than that, and it seems a little unlikely that a team would be boneheaded enough to have an O-lineman set off the line. What are the other requirements that a team commonly makes a mistake on, drawing this flag?
At the snap, you must have exactly 7 guys on the line of the scrimmage, no more and no fewer. The two on the ends are eligible receivers, the other 5 are ineligible. If you put a player with an ineligible number at the end of the line, you must declare him eligible or it's an illegal formation penalty.
If you have more than one WR/TE on one side of the line, one and only one of them must be on the line: the others must be behind it.
The most common illegal formation penalty is having 8 guys on the line, with a WR covering a TE or another WR on one side of the field. One of them screwed up by not stepping back enough.
On many plays, you can see the outside WR look to the sideline official and put his hand out to confirm that he's okay (not too far behind the line).
The OTs often line up a step or so back: the officials give some leeway, but you do see it called; it's the second-most common illegal formation penalty. My understanding is that they'll usually warn them before they start calling it.