Being an indifferent speller myself, I sympathize:
The occasion was the "Silicon Valley Leadership Group's annual business climate summit", and according to the SF Chronicle's Politics blog,
Duffy Jennings, spokesman for the Silicon Valley Leadership Group, said the banner came from the Clinton campaign.
I was going to ask, "don't big-time politicians have handlers on the look-out for stuff like this?" But I guess the problem is that Silicon Valley hasn't yet invented a spell-checker for banners. The technology is there, of course -- you could start with the cell-phone-camera translator, and just swap in a spell-checker for your ambient environment.
I'd rather not, myself; but then, I can rely on Geoff Pullum to point out my (frequent) typos and (occasional) spelling errors, and I've never been splashed across the media of several continents in front of a multiply misspelled banner.
[For more on the psycholinguistics of doubled letters in English spelling, see:
"Liberal gemination", 6/8/2004
"
Orthographic metathesis?", 4/1/2004
"
Jeniffer afficionados", 3/30/2004
"
The perils of degemination", 3/29/2004
"
Conservation of (orthographic) gemination", 3/29/2004
]