Never type Google into Google, because... you. can. break. the. internet.

Never steal from a thief and try to sell the stolen goods back to them at a later time.

Never use rattlesnakes as alarm clocks.

never date a spy

...forget to fill up the glass on the table with fresh, cold water, or the four legged hairy demons will make you suffer...

^ Never mention those particular demons in public.

Never play audio (music) loud through headphones via your phone in a public space, without first checking it's not coming out from the phones speaker, as you may receive unexpected looks... :D

Especially if it's from "those kinds of sites". o.0 ;)
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My contribution: Never waste time complaining about something a business does on it's forums(unless one has a large group to support it).....ones time is better spent gaming/taking up a hobby....almost anything else really.

And, of course, never use an apostrophe when using the posessive form of a pronoun. The apostrophe is used for noun posessives, but not pronoun posessives. An apostrophe with a pronoun forms either a non-word or a contraction. (For example, "it's" forms the contraction "it is", not the posessive form.) I know of no exceptions to this rule.

(Of course, this only applies to English; other languages have different rules.)

Never ask for more.

Never say "them motherfuckers" -- it will make you sound ignorant. Use "those motherfuckers" instead.

Never ask for ROGER Moore

(everyone knows Connery is best Bond :))

Never court Mandy Moore. She's taken.

Never pet a pet you've just scorned.