Personally, I tend to be brutally honest in quite a few cases and I wish I could avoid it. A lot of the time someone will ask me something, I'll reply instantly with the truth because it's the first thing that comes to mind and I usually don't censor myself if the situation doesn't require it.
A recent situation was that me and 2 friends were leaving school to hang out at A's house. B is away, and his girlfriend (C) asks if we mind her coming along. Since we're going to A's house, he replies, "No, come if you wanna". Being the unthinking asshole I sometimes am, I immediately said, "Alright, I'm just gonna stay here then". C asks "Awww, why?".
At this point, I noticed A was looking at me kind of funny, but, ever persistent on making an ass of myself, I said "Because you annoy the shit out of me" without skipping a beat. She suddenly looked incredibly dejected, and I felt like shit because I said it because I was in a shitty mood more than anything. B was understandably pissed at me, and I learned my lesson.
If you're interested we've patched shit up since then and all is well between everyone