It’s not melaveh malka during the summer.
It’s not Shaleshudis during the winter, either.
It’s not even metzitzah be-peh for a middle-aged convert.
It’s from Devarim 18:13, the passuk of ‘Tamim tihiyeh im Hashem Elokekha’, especially in the manner that the Rambam understands it. Here’s what he writes in the Laws of Idol Eworship, 11:17, at the end. Throughout Chapter 11, he discusses those things which we would call ‘magic’, basically anything that you would find in a Harry Potter book – divination, necromancy (although there’s none of that in HP, otherwise Harry would be able to contact Sirius), sorcery, and magic, all of which are prohibited by Jewish Law. Here’s how he concludes the chapter:
ױכל המאמין בדברים האלו וכיוצא בהן ומחשב בלבו שהן אמת ודבר חכמה אבל התורה אסרתן אינן אלא מן הסכלים ומחסרי הדעת ובכלל הנשים והקטנים שאין דעתן שלימה, אבל בעלי החכמה ותמימי הדעת ידעו בראיות ברורות שכל אלו הדברים שאסרה תורה אינם דברי חכמה אלא תהו והבל שנמשכו בהן חסרי הדעת ונטשו כל דרכי האמת בגללן, ומפני זה אמרה תורה כשהזהירה על כל אלו ההבלים תמים תהיה עם ה' אלהיך.
Anyone who believes in these and similar things, and thinks to himself that they are real and full of wisdom, but that the Torah prohibited them, is a fool lacking in intelligence, and is in the category of women and children whose intellects are incomplete. But intelligent people of whole mind will know with clear proofs that all of these things that the Torah prohibits are not matters of intelligence. Rather, they are meaningless vanities which attract stupid people, causing them to abandon the path of truth. Therefore, when the Torah warns about all of these matters it says “Tamim Tihiyeh im Hashem Elokecha.”
In other words, according to the Rambam, there’s a mitzvah not to be naïve, stupid, or gullible. I’d have put this one in the Ten Commandments:
Thou shalt not be a moron.
Thou shalt use the brain that I gave you.
It’s also a great weapon to have in the arsenal. Whenever anyone suggests that something is silly but harmless and innocuous, it’s always good to quote the Rambam that it’s an issur de-oraysa. Thee are no excuses for being that dense. It’s always very comforting when I can transform my exasperation with imbeciles into religious zeal.
NOTE: I've edited the original version of one of my examples of a 'difficult' mitzvah. The three examples aren't actually mine; I heard them in the name of the Klausenberger Rebbe, believe it or not, and I doubt that he meant to mean anything other than an unexpected jolt, wherein lies the humor.