A large reason why sarcasm works is because there is a distinct tone of voice when a sarcastic comment/phrase is being made.
In text, all tone is lost (outside of CAPSLOCK). If you write a sarcastic comment in a text message or on a forum, don't be surprised when whoever is reading your comment takes what you say completely seriously. In some cases, sarcasm is easy to pick up when it's extremely obvious. However, when your sarcasm is more subtle, the tone is lost entirely and you're left with what appears to be a completely non-sarcastic statement to the third party reader.
This might seem extremely obvious, but people still have a hard time grasping this concept. If you type everything with the same wording you'd use when speaking it, the chances are that any underlying alternate meaning (or what you really meant to say) may be lost.
Alternatively, I also see people who make a controversial post and then attempt cover it up under the guise of "guys, that was sarcastic!". This line of thinking falls under the same category as claiming you were "trolling and you guys need to calm down, it was a joke".
Tips to prevent autism of this kind:
1. Use the Universal Standard Sarcasm Tag (/s) at the end of a sarcastic statement.
2. Make your taunting extremely obvious (even then some people won't pick up on it).
3. Don't try to cover up your mistake by claiming "sarcasm". Embrace your words fully or admit they were wrong.
4. Stop making sarcastic comments. You sound like a 14 year old/ character from a Disney sitcom (most often these are one and the same) when you say them.
This has been TheOneTrueDesticle and I love every last one of you. /s