I found several mistakes in Pokéarth. Sorry if this post is really long.
http://www.serebii.net/pokearth/johto/mt.silver.shtml#hgss-2f
Gyarados is the only Pokemon listed under Super Rod for the second floor of Mt. Silver in HG/SS, but the encounter rate is listed as 63%. This can't be right, since if Gyarados was really the only Pokemon there, it would have an encounter rate of 100%. The remaining 37% should go to Seaking.
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/route26.shtml#c
The listing for Headbutt is inaccurate. The Pokemon that can be found with Headbutt in Route 26 in the Crystal version are Ekans, Exeggcute, Hoothoot, and Pineco; not Spearow, Ekans, Aipom, and Heracross. In fact, the real data for Headbutt in Route 26 in the Crystal version is the same as the data for Headbutt that Serebii listed for Route 44 in Crystal. That Headbutt data is found here:
http://www.serebii.net/pokearth/johto/route44.shtml#c
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/route27.shtml#c
The listing for Headbutt is inaccurate. The Pokemon that can be found with Headbutt in Route 27 in the Crystal version are Ekans, Exeggcute, Hoothoot, and Pineco; not Spearow, Ekans, Aipom, and Heracross. In fact, the real data for Headbutt in Route 27 in the Crystal version is the same as the data for Headbutt that Serebii listed for Route 44 in Crystal. That Headbutt data is found here:
http://www.serebii.net/pokearth/johto/route44.shtml#c
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/route29.shtml#c
The listing for Headbutt is inaccurate. The Pokemon that can be found with Headbutt in Route 29 in the Crystal version are Exeggcute, Hoothoot, Ledyba, Spinarak, and Pineco; not Spearow, Aipom, and Heracross. In fact, the real data for Headbutt in Route 29 in the Crystal version is the same as the data for Headbutt that Serebii listed for Route 42 in Crystal. That Headbutt data is found here:
http://www.serebii.net/pokearth/johto/route42.shtml#c
http://www.serebii.net/pokearth/johto/route30.shtml#c
The listing for Headbutt is inaccurate. The Pokemon that can be found with Headbutt in Route 30 in the Crystal version are Exeggcute, Hoothoot, Ledyba, Spinarak, and Pineco; not Spearow, Aipom, and Heracross. In fact, the real data for Headbutt in Route 30 in the Crystal version is the same as the data for Headbutt that Serebii listed for Route 42 in Crystal. That Headbutt data is found here:
http://www.serebii.net/pokearth/johto/route42.shtml#c
http://www.serebii.net/pokearth/johto/route31.shtml#c
The listing for Headbutt is inaccurate. The Pokemon that can be found with Headbutt in Route 31 in the Crystal version are Exeggcute, Hoothoot, Ledyba, Spinarak, and Pineco; not Spearow, Aipom, and Heracross. In fact, the real data for Headbutt in Route 31 in the Crystal version is the same as the data for Headbutt that Serebii listed for Route 42 in Crystal. That Headbutt data is found here:
http://www.serebii.net/pokearth/johto/route42.shtml#c
http://www.serebii.net/pokearth/johto/route31.shtml#4th
It says in the Headbutt section of Route 31 in HG/SS that Pineco can only be found in Route 31 in HeartGold. Pineco can be found in Route 31 in HeartGold and SoulSilver.
http://www.serebii.net/pokearth/johto/route33.shtml#c
The listing for Headbutt is inaccurate. The Pokemon that can be found with Headbutt in Route 33 in the Crystal version are Spearow, Ekans, Aipom, and Heracross; not Exeggcute, Hoothoot, Ledyba, Spinarak, and Heracross. In fact, the real data for Headbutt in Route 33 in the Crystal version is the same as the data for Headbutt that Serebii listed for Route 32 in Crystal. That Headbutt data is found here:
http://www.serebii.net/pokearth/johto/route32.shtml#c