In other words "there is no such thing as 'being too careful'". You can be too loud. You can be too rude. You can be too violent. But there is never a time when you can be accused of being too careful. It is not possible to be too careful. Being too careful is not a thing. It's not to be taken strictly literally, though. It means "up to a certain point, it's better to be more careful than less careful" or "an overabundance of caution is preferable to too little." In fact, it actually is possible to be "too careful". But nobody ever is. :D

跟中文成语 ”不怕一万，就怕万一“ 差不多了。This is similar to the Chinese phrase, 不怕一万，就怕万一。