In Sundog: Frozen Legacy (Apple II, 1984), we had a fairly robust, multi-level copy protection method. However, many of the 'cracking' tools out at the time would actually produce a runnable copy of the game -- it was just that the game wouldn't pass its final internal DRM check. In the game, including in 'cracked' versions, you started out on the surface of a given planet (Jondd); you could drive around the planet's surface, walk around the cities, go into stores, buy and sell goods, etc. But when you attempted to lift off into space, if that final DRM check failed, you'd get the message "Clearance to lift denied due to pirate activity" and you would be unable to take off and travel to any other world or system. (Note that you'd never see that message in a legitimate copy of the game.)
Now, the game actually had space pirates who would attack your ship, so a lot of people didn't realize just what the message meant. We would get occasional phone calls from customers asking what they were doing wrong and how they could get clearance. We'd listen for a minute, then say, "Well, just mail us your Sundog floppy disk, and we'll send out a new one for free." Heh. On the other hand, we had at least one person call us up on the phone and say, "Yeah, I get it" and then order a legit copy.
Note that for those customers who did buy an actual copy of the game, if they sent in $10 along with their registration card, they'd get another Sundog floppy disk -- that is, a second complete copy of the game, which they could keep as a backup or give away (or, frankly, sell). Also, if anyone actually did have a legit Sundog floppy that died or was otherwise damaged, we'd exchange it for a new one for free.
Sundog (Apple II) was on Hardcore Computing's "Top 10 Wanted" list (for a cracked version) for quite some time. It was eventually cracked, but I believe it took a year or two. You can find runnable Apple II disk images (for Apple II emulators) online.
I really don't know what copy protection was in place for the Atari ST port of Sundog, since that happened after I left FTL Games.