What's the catch on houses that are in pre-forecloser?

No catch really.
When a lender notifies a mortgagee that unless their back payments are all paid up by a certain date they will be foreclosed.

This should be a big signal to the mortgagee that he need’s to meet with the lender and negotiate a new situation. The lender doesn’t really want to end up with a house, because they then have to find a buyer.

If you happen to know of someone in pre-foreclosure, there is nothing wrong with talking to the borrower about their plans.

The mortgagee can only accept the lender’s option(s) if they can actually meet the obligations, or else they are foreclosed and have a ruined credit history which can dog them for many years.
You can agree to buy the house from them at conditions that meet the banks offer.
This means the mortgagee will still leave (unless you choose to rent to them at a price they can afford), but they won’t have the damage to their credit because you have stepped in to satisfy the bank.

In short, you are responsible for the mortgage, but the mortgage may be less than the original mortgage because any options the lender may have offered during the foreclosure period.

Time is of the essence as the period between notification and foreclosure is short. Based on news articles recently, many people are being foreclosed without the benefit of the pre-foreclosure period.