Many people simply do not realize that written math has notational conventions, and despite the fact that the algorithms and proofs are logical and complete, the notation is just a language and has holes and ambiguities like anything else.
As plenty of troll questions on Facebook show, many people will defend their interpretation of "what is the value of 5 + 1/2(6)" to the death, acting like PEMDAS is a complete mathematical rule handed down by the gods unto man and not just an incomplete (doesn't include juxtaposition!), arbitrary convention that makes it easier to figure out what some other person wrote.
The best way I've ever seen this solved is just to write a bunch of really easy but ambiguous expressions on the board. Tell the class to solve them on a sheet of paper, have them hand it forward, and then take a tally of how many completely different but still correct answers you get for each problem. Then use this as a learning experience about how math requires clear communication just like everything else, and that despite it being a logical field, not everything is an invariant theorem so you really don't know what they mean when they write confusing and ambiguous expressions.