the iphone does have a feature that automatically extends the touch area of a letter that covers up some neighboring letters in cases where it recognizes the word before you finish typing it. Making it less likely to make a mistake on the last letter. So like for MacBU for whatever reason it thinks you are spelling MacBI so after you type the "B" it extends the touch area of the "I" over the "U." Don't see why it would recognize either word but I have been able to replicate this on some other common words.
Here are some other words where it extends the touch area on the last area: Banana, Short, About, Most, iphone, etc.
In those cases it's been impossible to type an adjacent letter on the first attempt at the word. But can be solved by hitting the delete key.