Allaah (The Most High) said:
وَلَا تَلْبِسُوا الْحَقَّ بِالْبَاطِلِ وَتَكْتُمُوا الْحَقَّ وَأَنتُمْ تَعْلَمُونَ
And mix not truth with falsehood, nor conceal the truth while you know (the truth). [2:42]
Imaam Ibnul Qayyim (rahimahullaah) said: Allaah forbade (us) from mixing truth with falsehood, and thus leading to concealment of the truth. Mixing truth with false is by mixing the two until one of them is confused for the other. This is forgery and deceit by way of which what is made apparent is the opposite of what it is in reality. Likewise, when truth is confounded with falsehood, the culprit manifests falsehood in the image of truth and speaks with a statement that carries two meanings- a correct meaning and false meaning, so the listener is under the illusion that the speaker intends the correct meaning, whilst he (i.e. the speaker) intends the corrupt meaning.
[Source: Badaa’i At-Tafseer Al-Jaami Limaa Fassarahu Al-Imaam Ibnul Qayyim’ Vol 1, page 124. Abridged and slightly paraphrased]