I'll just skip over Crystal-related errors for now.
http://www.serebii.net/pokearth/kanto/route26.shtml#gs
In the listing for "Headbutt on Special Trees", it lists Kakuna as having a 5% encounter rate in Silver. Actually, that should be Beedrill there, not Kakuna.
http://www.serebii.net/pokearth/kanto/route27.shtml#gs
In the listing for "Headbutt on Special Trees", it lists Kakuna as having a 5% encounter rate in Silver. Actually, that should be Beedrill there, not Kakuna.
http://www.serebii.net/pokearth/kanto/powerplant.shtml#4th
You forgot to mention the Dugtrio-for-Magneton trade in HG/SS.
http://www.serebii.net/pokearth/johto/route31.shtml#4th
In the listing for Headbutt, it says that Pineco can only be found on Route 31 in HeartGold. Pineco can be found on Route 31 in both HeartGold and SoulSilver.
http://www.serebii.net/pokearth/johto/route39.shtml#4th
In the listing for Headbutt, it says that Spinarak can be found on Route 39 in SoulSilver. Spinarak is exclusive to HeartGold; that should be Ledyba there, not Spinarak.
http://www.serebii.net/pokearth/johto/route46.shtml#4th
In the listing for Headbutt, it says that Ekans can be found with Headbutt on Route 46 in HeartGold and SoulSilver. Ekans can only be found with Headbutt in Crystal; that should be Spearow there, not Ekans.
http://www.serebii.net/pokearth/johto/burnttower.shtml
It's actually called the Burned Tower, not the Burnt Tower. In fact, it always was called the Burned Tower, even in GSC.
http://www.serebii.net/pokearth/johto/thecliffcave.shtml
It's actually called "Cliff Cave". The word "the" isn't in the name of the location.
http://www.serebii.net/pokearth/johto/cliff'sedgegate.shtml
It's actually called "Cliff Edge Gate".
http://www.serebii.net/pokearth/johto/goldenrodcity.shtml#4th
The Spearow that you get is actually obtained in the gatehouse, which is part of Route 35 (I checked with the Pokegear map).
http://www.serebii.net/pokearth/johto/goldenrodunderground.shtml#4th
In HG/SS, the Goldenrod Underground is called the Goldenrod Tunnel.
http://www.serebii.net/pokearth/johto/shininglighthouse.shtml
It's just called the Lighthouse, not the Shining Lighthouse.
Finally, the main page of Pokearth still links to the Hidden Tower instead of the Embedded Tower.