Very cool quiz! Usually I don't enjoy much the 'english language' category as it's not my first language but this one was more accessible yet still fun.
Not sure these are right but what about "'bout" (maybe it's slang?) and "one's" (as in 'one is')
Glad you had fun! There are (especially in poetry, songs and conversation containing lots of slang) many more contractions used like 'bout for about, and I agree with you that 's can be added onto nearly anything as a contraction of "is" - for example, "that bird's flying fast". Neither 'bout nor one's are listed in dictionary.com, although where's, for example, is. My guess is that the latter is included in the dictionary, despite the omission of the others, because it's in more common usage.
Not sure these are right but what about "'bout" (maybe it's slang?) and "one's" (as in 'one is')