It obviously depends.  If the official donation receipt is in some small way incomplete it could be a penalty equal to 5% of the eligible amount on the receipt. However if it is deliberately false information then it can be a penalty of 125% of the eligible amount on the receipt.  There are also some people sitting in jail right now for issuing fraudulent receipts.  Here is a note from CRA on this point.

http://www.cra-arc.gc.ca/chrts-gvng/chrts/prtng/rcpts/cnsqncs-eng.html
Consequences of improper receipting
What are the consequences for charities that issue receipts with incorrect or false information?
• A registered charity that issues an official donation receipt that includes incorrect or incomplete information is liable to a penalty equal to 5% of the eligible amount stated on the receipt. This penalty increases to 10% for a repeat offence within five years.
• A registered charity that issues an official donation receipt that includes deliberately false information is liable to a penalty equal to 125% of the eligible amount stated on the receipt.
o If the charity is liable for penalties in excess of $25,000 for issuing false receipts, that charity is also liable to one year’s suspension of its charitable status.
• A registered charity that contravenes or continues to contravene the receipting requirements of the Income Tax Act could also have its registration revoked.
Related Topics
• Guidelines for Applying the New Sanctions
• Penalties – Suspensions Chart
References
• CSP-R02, Receipt
• CSP-F17, Penalties
• CSP-S18, Suspensions