If you are talking about a 3rd party coin grader (PCI, NGC, PCGS, etc.) the likelyhood of that happening is next to nothing. If you are talking about a local coin grader/dealer, it might happen and here are a couple of things you can do to protect yourself (this can be done for either of the above graders):
1. Take a good digital photo of your coin (Obverse and Reverse). Note any unique marks or discolorations.
2. If dealing in person, ask the dealer to photograph the coin and present a detailed receipt. If submitting a larger quantity of coins, prepare a detailed inventory of the coins and have the dealer sign it as having received the coins. If he balks at this, I would find another dealer/grader.
3. Question any discrepancies IMMEDIATELY upon receipt of coins. If in person, go through the coins before leaving the store. If through the mail, withing 1-2 days and contact by phone.
This may be a little time consuming, but it will protect you in the long run.
MGirard