"Clearly, this is true."

[elwen]

If you've ever been annoyed by professors that say, "Convince yourself that this is true," or books that tell you, "The proof is left as an exercise for the reader," here's even worse:

Because these statements are intuitively acceptable, they are presented without proof.

It's especially effective because the previous sentence says they can be "easily proved". But, apparently they don't feel like it anyway.