"Developement" is not a spelling error, it is an alternative spelling for "development", anon user 153.42.34.240.

No, it is a spelling error. At any rate, it's not English: see M-W.

One source? Very weak, and sadly, you are mistaken. See for instance:

1. "developement." Webster's Revised Unabridged Dictionary. MICRA, Inc. 18 Jan. 2008. <Dictionary.com http://dictionary.reference.com/browse/developement>.
2. "developement." The Collaborative International Dictionary of English v.0.48 [gcide]. 8 Jan 2008, 17:01 UTC. <dict://aspen.miranda.org/d:developement>
3. "Developement." Wikipedia, The Free Encyclopedia. 8 Jan 2008, 17:01 UTC. Wikimedia Foundation, Inc. 18 Jan 2008 <http://en.wikipedia.org/w/index.php?title=Developement&oldid=182978942>.

Furthermore an occurence count of 1,850,000 in English language sites (google) would indicate that, although not as frequently used as the dominant spelling "development", it does occur significantly. Now stop wasting my time by wrongly arguing about inanities, get a non-anonymous id, and make a real contribution.