Why did Lego use the same number for two different sets? For example, Ricky Racoon on his Scooter from 1979 is a completely different set from House with Garage from 1996, but they share the same number (324). Why did Lego recycle numbers like this? Do they contunue to re-use old numbers?
I recall someone asking this to a Danish employee at a public event in the US around 2003. In a very humble voice the LEGO employee simply said "We forgot." LEGO employees were admitting before that to using brickshelf for old instructions because they didn't have digital company archives yet.

It's possible they knew about the numbers and saw no problem with reusing the retired number. It's we obsessive fans that are the problem! When I was a kid, making my first LEGO master catalog, I was upset to discover 540 had been reused, for Swiss Villa, 1973 and Police Units, 1979 (before they assigned it to Basic Building Set, 1985. Argh!)

You can find many examples with this Peeron search Change the -2 to -3, etc to find more layers. Open a set, then edit the URL to -1 to see what it clobbered (sometimes Peeron has the newer set inventoried first as -1.)

So you can see how they are still doing it. But mostly it is done to impulse items, promos, basic bricks, and non-brick lines. I can't find a core set bumping off another set number after 2000 (see Scorpion Buggy for example.) Maybe they have stopped after that?