Proof by intimidation Trivial!
Proof by cumbersome notation The theorem follows immediately from the fact that
Proof by inaccessible literature The theorem is an easy corollary of a result proven in a hand-written note handed out during a lecture by the Yugoslavian Mathematical Society in 1973.
Proof by ghost reference The proof my be found on page 478 in a textbook which turns out to have 396 pages.
Circular argument Proposition 5.18 in [BL] is an easy corollary of Theorem 7.18 in [C], which is again based on Corollary 2.14 in [K]. This, on the other hand, is derived with reference to Proposition 5.18 in [BL].
Proof by authority My good colleague Andrew said he thought he might have come up with a proof of this a few years ago...
Internet reference For those interested, the result is shown on the web page of this book. Which unfortunately doesn't exist any more.
Proof by avoidance Chapter 3: The proof of this is delayed until Chapter 7 when we have developed the theory even further.
Chapter 7: To make things easy, we only prove it for the case
Appendix C: The formal proof is beyond the scope of this book, but of course, our intuition knows this to be true.