Arbitrage
I'm aware of the theory behind this and for any single event you can guarantee profit. But can the same effect be ashieved over a long period where on average the prices are in your favour. For example, if you had a two horse race with one at 1.8 and the other at 2.5 you could bet £5.55 on the first and £4 on the second and gusrantee a gain of 45p - around 4%. This is a totally artificial situation but, could you achieve the same result and almost (allowing for probabalistic inbalance) guarentee the same 4% gain if you put were you get an equal amount on each horse at those same odds over, say 100 races? Does the fact that the person selling those bets at an effective loss over a prolonged period of time mean you could take advantage of this by just betting equal amounts on each horse?