I am looking for a general answer; nothing too detailed. And no. This is not a homework question. It happened to me in lab and I am wondering what error I could have committed.
Below is a basic example.
If percent yield is based on a mass, then a >100% yield can result from a product contains materials that should not be there. For instance, a product that should be anhydrous may not be completely dry and water would add mass to the product increasing your yield. Other present contaminants can also do this and can even appear from a product that reacted with the surrounding atmosphere.
Preface: The problem is that you have overstated your percentage yield, and the symptom is that it is above 100%.
Example: You and your friend each do an experiment where the literature states the yield should be ~65%. You both use poor technique and fail to dry your product, causing you to attribute water mass as product mass. Your friend's yield is 101%, and yours is 99%. Both you and your friend are wrong, for the same reason! Don't make the mistake of stopping your analysis when percentage yield drops below 100%. Don't chastise your friend for having an "impossible" yield while patting yourself on the back for your "99%"!
Overstatement of percentage yield happens for the following reasons:
Zero) Your model of the reaction chemistry is just wrong. Unlikely in an undergraduate setting, but worth mentioning.
1) Your math is wrong. Worth double checking and easy to rule out as a source of error.
2) You used more reactant than you thought. Most errors in reactant measurement cause you to use less reactant than recorded (e.g. transfer errors, impure reactants), but you could possibly have used more than you thought. Beyond simple measurement errors, perhaps your limiting reagent was provided to you in solution, and that solution was of higher concentration than reported. Put a bit of time into thinking about this, but [especially in educational experiments] protocols usually minimize the chance of this kind of error.
3) You actually have less product than you think. This is the most likely culprit. You have to be really honest with yourself here. In most experiments, you take the mass of something at the end, and call it the mass of your product. Is it really your product? Or does that mass contain impurities which cause you to misrepresent impurities as product. Likely culprits are solvents (e.g. water from improper drying), side-products, and/or unreacted reagent. Have you really done your best to filter and remove them? If not, your yield will be inflated.
Yields above 100% are obviously impossible, so either you used more educts than you thought, or you don't have as much product as you think.
An error when measuring the amount of educts for the reaction is one possible reason. A miscalculation at any point of the amounts needed or the final yield is also something you might want to check again.
A really common reason is the presence of other material in your product, often solvents like water are still present if you weren't careful enough when drying. This is really the most likely cause, especially in student labs.
You should think about the analytics you performed, and check if they show the presence of anything aside from your product. And then think about what kinds of substances could be in there (solvents, educts, other reaction products) that wouldn't be visible in the kind of analytics you performed.
Percentage yield exceeds 100% if impurities are present due to inadequate purification. Such impurities could be present for many reasons, such as if the vessels used to collect the product are contaminated. Also, it is possible that the product absorbs components from air like water vapor or carbon dioxide.
Like others said, contaminants may be there or unreacted chemicals can be there. It happened to one or two times while I was doing summer project and left the compound in vaccumm and a trap to get rid of solvents. You should use proper solvents to separate the product and unreacted or side product stuff.