There are a number of words which are forbidden and will be replaced with asterisks - basically what used to be called 'four-letter words' and also a few things like 's-t-r-e-a-m-i-n-g' (because of a spate of spam linking to such sites) and 'p-o-r-n' for obvious reasons (of course I can't type the actual words, hence the hyphens). There are a few other things, some of them glitches - for example, I don't know if it's still the case, but it used to be that if you placed a semi-colon on the end of a URL it would be rejected.
Occasionally you will get a banner saying you can't post content, which is just a glitch and goes away without having any effect. If you get a notice while actually posting you should look at your content and see whether anything in it is likely to provide this.