Clearly, lying is inappropriate, and don't do it!
However, failing to submit a transcript from a school can never be found out. If you fail to submit your freshman grades from a college, clearly, AMCAS is going to wonder what you were doing your freshman year, and this will raise flags, causing you to be in deep trouble.
However, if you fail to submit transcripts from "other" colleges that you "tried" courses on for size but did poorly, AMCAS will never know. For instance, I know someone who took organic several times until she received an A but did not submit her previous "attempts" which were not at her home school. While I don't agree with her, AMCAS will not find out about her because they have no way of knowing that she took the classes. According to the Family and Education Privacy Act of 1974, educational records are sealed and only the student can release them, which is why students must request their transcripts be sent. If the student fails to submit some transcripts to AMCAS, it would be illegal for AMCAS to obtain the transcripts EVEN IF THEY KNEW THEY EXISTED. But AMCAS would have no way of knowing such transcripts even existed unless the student disclosed that. AMCAS is powerful but anemic when it comes to this issue. Interestingly, some schools maintain "internal" records that are not disclosed to AMCAS. I know of at least one Ivy which openly declares that it will not disclose classes students fail. AMCAS is aware of this policy and cannot do anything about it. The only way AMCAS could obtain these records would be to break into the school's system which would be highly illegal.
Given this policy, do you think AMCAS should sue the Ivy that I'm talking about and force them to disclose? What legal basis would AMCAS have for such a suit? Would they hire a Wall Street firm to litigate? Would the Ivy, which has a larger endowment than AMCAS, win the suit? Is AMCAS more powerful than the FBI, Harvard, Yale, Princeton, and the CIA all put together!