Nowadays all websites are very strict to spammers. People spam more and more (I reported a new Facebook scam in this month). Facebook is exceptionally strict to uncommon behaviors. They are really concerned about users’ privacy. Therefore there are more possibilities that you can get banned or your account gets limited features, if any kind of uncommon behavior occurs by your end.
You have to go through spam filter if Facebook guesses you are trying to spam. You should be serious about this spam filters. If you keep entering wrong characters your facebook account gets banned or limited quickly.
The best thing to do when those words are not clear is to click on “Try different words” because sometimes those captures are not readable. Remember Facebook is fully systematic whereas there is no human involvement in their procedures. Once your account gets banned, it’s impossible to get your account back.