Dr Ron, you are correct.
You lay the horse at 3.75, and back it at 10 with the bookie.
To maximise this situation (50 pounds = approx $120), you would then have $45 on with the bookie for a rough profit (commission etc) of $71.

However, La Mer is also correct, these are few & far between & is obviously someone's mistake, and his example of Super Elegant is a beauty. You are more likely to lay at 3.75 & back at $3.9, if you're lucky.

When Betfair first started there were mistakes all over the place, it was just a matter of being johnny on the spot to grab them. Don't feel guilty about taking advantage as you will make mistakes as well, as we all have. I once made a book to 135%, in one go!, but have also laid horses @ 500/1 when I only wanted to 50/1.