Heh, I guess just a lack of confidence, but also after I typed it out I thought that it's a bit of a strange thing to say.
So, ほら! is like "just like I thought/here it is!" then? I was thinking something like "hey, a proofreading error! What, was the proofreader sleeping or something?" (that is, just discovering it and not having been expecting it). I was shooting more for surprise than anything else, maybe with a little bit of annoyance. So おい! would be a better word choice there?