Interestingly, while humor/humour is a US/British difference, humorous/humourous is not, the latter is just rare/unusual.
* https://www.google.co.uk/search?q=unshakeable (even makes the mistake on the UK Google site).
Or "misspelt/misspelled", as we prescriptivists like to call it.
Though when I see hypercorrections like "humourous" on the internet, I begin to suspect that people are using UK/Commonwealth spellings because they think they should, not because they're actually more familiar with those spelling conventions.