Censorship in wartime is nothing special. What happened with this postcard from the occupied Netherlands (31-05-1940) to Batavia, Dutch East Indies, is therefore not rare. The two-line red stamp “Gecensureerd” (censored) is very common.
The back (and also the front) was completely used by the writer, but the majority was scratched away and is therefore unreadable. The removal of text by the censor is nothing special, although it is usually done with black ink or – for a letter – by cutting out the relevant piece.