Missing comma in a series
Three or more words or phrases functioning as a list in a sentence must be separated by commas:
Al opened the refrigerator took a big whiff and quickly shut the door.
Problem and cure: To head off a misreading, simply place a comma after "refrigerator" and (if desired) after "whiff."
This is simple stuff, granted, but many writing errors are the result of carelessness. Though the second comma (after "whiff") is optional, many writing professionals strongly recommend including it. Whatever the case, be consistent.
Sometimes lists are composed of long or potentially confusing phrases that are better indicated by the use of semi-colons instead of commas:
My mother gave me some instructions: go to the bank and deposit the check; stop by the store and pick up a gallon of milk, some bread, and a watermelon; drop in on Grandma and make sure her air conditioner is running; go to the newsstand and buy a newspaper; and make sure the cat has some food and fresh water.