I don't know if this holds true for all phones, but I've accidentally saved a few goofy "words" to my phone's autocorrect. It comes up as an option every time I type something in, including things like: I tried to type 'bigger' and got 'biggest' from my phone, so I backspaced and corrected it. Somehow, I added 'gger' to my custom dictionary as a result, and now it comes up a lot as a suggestion. Also, if I miss the keyboard while trying to 'swipe' in a word, it will sometimes sub in some really odd things as well.
It does learn a little bit - it capitalizes the names I type in frequently, and it offers my husband's name as a suggestion before some other close words. But sometimes I think it just likes to mess with me, and refuses to recognize words out of spite.