In order for this tired, old riddle to make any sense, you need to post the whole thing as this asker has done:
Find the error. It's impossible!
Did you know that 80% of Stanford students could not find the error above?
After the list of letters, the riddle goes: "Did you know that 80% of Harvard students could not find the error above?" This line is an essential part of the riddle--if you leave it out, it makes no sense.
(Note that the name of the school varies from telling to telling.)
The error is the phrase "it's impossible," since 20% of the students DID find the error.
To summarize, this riddle has 3 parts:
1. A statement that it is impossible to find the error.
2. A list of letters, numbers, math equations, etc. which does NOT contain any errors.
3. A statement indicating that at least one person was able to find the error.
hope this helped you