I find it interesting that you would slip it in their morning coffee because they are more than likely physically dependent on that morning caffeine. As for your question, you obviously would experience all the withdrawal symptoms found in any physical addiction. There would probably not be any psychological addiction because they are oblivious to the fact. The only way you could practically perform this experiment would be to introduce very minute portion of the addictive substance and continue with incrementally larger doses. If you slipped .5g of cocaine in their coffee the person would certainly notice the difference! I think the results are fairly predictable. Interesting question though.