Many times new users do not know how to correctly hide spoilers by using the corresponding tag (>!). They often just write something like "SPOILER ALERT" or some variant. One user attempted to obfuscate the spoiler by writing it backwards.
If these people knew there was a better way of showing spoilers, I'm sure they would use it. I suggest that when the editor detects the word "Spoiler" in an answer/question, it pops up something at the top telling the user how to use spoilers