While trying to remember the word tmesis, I found the wikipedia entry for shit, which contains this paragraph:
Non-native English speakers should take note that shit and fuck often serve different uses as expletives, such that (for instance) the gerund, shitting, is rarely used emphatically. Ex.: In the sentence, I was so shit-scared of that shithead that I shit-talked him into dropping out of the shitting karate match, the phrase, shitting karate match, would be incomprehensible to native speakers except in suggesting a singularly unsanitary form of karate. (In the UK, phrases such as shitting hell as an emphatic are not unknown.) A correct and clear vulgarism would be, the fucking karate match. Similarly, shit is never used as an infix: While in-fucking-credible is comprehensible English, in-shitting-credible is not. Shit you! is likewise a puzzling and ineffective expression of defiance. It is not uncommon, however, to encounter an adjective constructed partially of the word shit, such as “Shittastic” or “Shittacular”.