I say some stupid stuff.

Not just in Japanese--this happenes in every language I've ever spoken in.

I think the main problem is "speaking before thinking."

In English, this is because I'm living in outer-space fantasy dream world. In Japanese, though, I've found that I simply can't keep up with the pace of my own sentences.

Mouth: fast.

Brain... slow. *_*

The result is that I often say something, stupid, offensive, and/or regrettable.

Luckily, I have one secret-weapon phrase to keep everyone from hating me when I make a slip:

いい意味で
ii imi de
in a good way
(Literally, "in a good meaning")

You can tack this phrase onto the end of any easily misconstrued sentence.

お前バカだね（いい意味でね）
omae baka da ne (ii imi de ne)
You're an idiot. (And I mean that in a good way.)

めっちゃ辛い！いい意味でね。
meccha karai! ii imi de ne.
This is so spicy! In a good way.

My favorite one is this blog title that I found on Ameblo:

君って、鯖に似てるよね。いや、もちろんいい意味でね。
kimi tte, saba ni niteru you ne. iya, mochiron ii imi de ne.
You kind of look like a fish. No, no, in a good way, of course.

(Note: He actually says 鯖 (saba), which is a type of mackerel, I believe. It's a popular dish in Japan. But I thought "fish" sounded better. Also, if it makes you feel better, at first I didn't recognize this kanji for saba.)

(Note #2: When he says いや (iya), it means something like "no," but I get the nuance that the listener made a less-than-pleased face just before he said it: (1) You look like a fish. (2) Listener looks upset. (3) No, no, in a good way, of course.")

Looking on Ameblo, it appears that もちろん (mochiron), "of course," comes before this phrase quite frequently.

日本語って難しいよね。もちろんいい意味でね。
nihongo tte muzukashii yo ne. mochiron ii imi de ne.
Japanese is hard, huh? In a good way, of course.