Is it legal for a supervisor to punch an employees timecard at the end of day if the employee forgets to do so?

Where are you?
Different states have different regulations regarding these things.
Also, what type of company is this? A privately owned business, a stocked company (can't think of the term right now... xP), unioned?
All of these things can change the outcome of the answer (as far as I know, depending on where you are).

Helpinhere, could circumstance effect it too? I'm in CA and anytime me or one of my coworkers forgets to clock in or out we just let a manager know and they go into the computer and fix it for us. I've always assumed it was legal. So I would have thought the answer to dtruch's answer was "yes." but maybe its different if the employee doesn't say anything?

I also live in CA. I know at WalMart, you aren't allowed to do that. However, if the timeclock (what they call it... sounds kind of stupid to me) has a line, or it's busted, or etc. you have to go to a manager, and they do it all manually (checking in or out).
There's more I could go into, but a lot of it is WalMart specific, xD

I know that privately owned businesses, unless there is some state/local law against it, can do the time however they want, and it is up to the owner(s) to take action if anything should occur. Just "letting a manager know" may not always work, because, depending on the regulations put into place, the law/policy may not allow that.

I do work for a privately owned company. I just re-read the time clock page in our handbook. It says to let a manager know if we ever need to edit our time card for any reason. So I guess that's what makes it legal for us