This is really weird - just earlier tonight at our game session I was telling people about the Comeback Inn and also couldn't remember where I'd seen it. Another player talked about the Welcome Inn, which he and I both kind of remembered being in the Blackmoor part of Oerth (same world as Greyhawk's on) and as being a planar nexus of some kind; that might be what [MENTION=23935]Nagol[/MENTION] is referring to.
So does that mean in effect that the Welcome Inn and the Comeback Inn are the same thing/place?Adventures in Blackmoor is D&D Expert module DA1 (Dave Arneson 1) and is set in Mystara / Arneson's Blackmoor (assigned as Mystara 3,000 years ago) as opposed to Greyhawk's homage location. The mostly indestructible inn contains both a temporal and dimensional gate though the PCs only experience the temporal aspect first hand, dangerous planar critters leak through demonstrating the other. The entire adventure takes place inside different incarnations of the inn -- at least 3 and more likely 6-8. There are 3 distinct versions and one version you can end up visiting in the recent future/past a few times.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.