Heretic: Shadow of the Serpent Riders came out a while after Heretic had been released as Shareware. Two years by my count, checking Mobygames. I remember as a kid I'd played the Shareware episode to death. But actually purchasing Shareware games was a no-go since it involved giving my parent's credit card out over the phone, or mailing a check for a bank account I didn't have. It was retail or bust for me as a young teen. Lawn mowing cash only.
So it was at some point post Quake 2 that I found the belated retail version of Heretic in a bargain bin or a bundle, probably at CompUSA, and snatched it up. I may or may not have even installed it. It mostly sat in my collection, a memory of all the fun I'd already had with the free Shareware episode.
Well I finally dusted off my old copy that I've kept with me ever since that day. And I played through it to completion, all five episodes.