Proactive interference occurs when OLDinformation stored in LTM affects recall when learning new information. This usually occurs when the old information is similar to the new information.
In the experiment the OLD information (List A starting with Apple) should have affected you remembering the new information (List B starting with Jacket).
An everyday example of proactive interference is when you are trying to remember the names of your new teachers and classmates: your memory for of your old teachers and classmates makes it difficult to remember the names of the new ones.