I think it was in March 1994 on Fishing Creek at Grassmere Park in the old special regs area. I went there because I got off work early, since I'd never fished there before I thought I'd check it out because I was nearby. Anyway there were plenty of recently stocked fish as evidenced by the number of trout I could see. I tried an old favorite, a Royal Wulff, and they simply ignored the fly in favor of following me around for hours. Thankfully this is now a wild trout stream and the brookies there spook fairly easily. But I assure you they aren't brookies from hatchery fish; these are purely wild strain fish. Since I don't fish stocked streams much I don't have this happen too often, but on Bear Creek in Schuylkill County just last year they were following me around, maybe I smelled good to them. Seems I never smell good to any humans, but that's cause I work hard and play harder.
Then there was the time at the Little Lehigh, after having purchased a bag or 2 of pellets, I fed some of the hatchery fish. Having finished walking around the hatchery and checking things out there I proceeded to walk the upper section of the fly stretch. I'm always interested in what is happening there, so I went up to where there was no one fishing because I wanted to see just how many fish were in one of my favorite pools. Anyway I threw in a handful of pellets and more trout then you can imagine came up and were fighting over the pellets. This section is officially unstocked remember.
So I got a bit more aggressive with the feeding and the entire pool was boiling with fish. I used up the trout pellets, and started walking again, down to the hatchery pool this time. I asked one of the guys there if he was having any luck and I got a rather terse, "I don't have time right now, there's a hatch and I've got to figure out what the trout are taking."
Absolutely true story, so the trout never forget what a pellet is, I promise that if you want to catch picky hatchery fish long after they were stocked, try a pellet fly.