Many iPhone users have found that a glitch is currently preventing them from typing the letter "I" in text messages. The device instead corrects the "I" to a random character like "A," "#," or "!". The bug also seems to affect typing on apps including Twitter and Instagram. Apple says it's working on a fix that will be released very soon. In the meanwhile, it suggests that you set your autocorrect of capital "I" to a lowercase "i."
Source: Buzzfeed