Undo an autocorrected word on the iPhone keyboard
There's a pretty simple way to undo any auto-correct on the iPhone's keyboard: if you type a word that autocorrects, just hit the delete key, and a popup of suggestions for that word will pop up. The first suggestion is the word you had before it was autocorrected. 

So say you type thid, which autocorrects to this, but you really meant to type thid. 
Just type thid, then hit space, then backspace, then a popup with the word thid will appear. Tap it to undo the autocorrection.

Here's a video.