Well I think it is possible to get very unlucky with your type of bet.

The Betfair Users guide quotes the case of every horse in a UK hurdle race falling - think a five horse field.

The first horse (coincidently the favourite) was legally remounted, completed the course and declared the winner.

Unfortunately one punter had laid it in running at 1000/1 after it fell.

I think you will get a very high strike rate and I guess if you don't go over 150/1 you may get a long run before your hit..............

But I've found with doing unportected lays you will eventually get hit - and your bank suffer accordingly.

That is why others have advocated a staking method of betting to a maximum liability with multiple selections.