Flush Draws

Moving along from Positive/Negative equity, lets take a look at it in action on a flush draw:-

A typical flush draw in holdem is 9 outs. (ie you have suited hole cards, and 2 cards of this same suit come on the flop, leaving 9 cards of that suit)

Now, you have a 35% chance of hitting one of your 9 outs if you are drawing from the flop.
So, if someone bets the pot (which will be a cost of 33% for you to call), then you have positive equity for a call ('I have the pot odds to call')
However, you only just have it. Therefore if the pot was say 3000 chips, and he went all in for his remaining 6000 chips, you should know immediately that this would be a negative equity call.

Once the turn comes, this changes. Your chance of hitting one of 9 cards is now only 19.6% (because only river to come). Therefore any bet in the region of the size of the pot should instantly ring alarm bells (to call a pot size bet is 33%). If you call here for this amount you are getting massive negative equity.
If the person above had checked the flop, and then launched his 6000 chips in on the turn, then obviously I do not need to state how bad a decision this would be to call. (Just to ram it home, it is a 40% cost for a 19.6% chance of winning).