Internet message boards are cauldrons of casual speech. They're often riddled with typos and grammatical errors: the one-word "alot," countless plurals formed with apostrophes as in "cappuccino's" and "luau's," the examples go on. (For the record, those should be "a lot," "cappuccinos" and "luaus.")

Normally, no one mentions these errors — even the message board users who obviously know better. So for a mistake to stand out in this sea of anonymous fast-typing and casual slip-ups, it has to be pretty bad. This one was: A user on a travel message board wrote that she was looking forward to "my husband and I's first trip."

Bad as that is, I wouldn't be mentioning it had I not noticed the user name of the person who posted it: EnglishTeacher702.

Now that's bad.

Of course, we can't know if she actually was an English teacher. But based on my years of talking to people about grammar, it seems likely. The English majors, English teachers and former English teachers who write to me often know a lot about Shakespeare and Austen and Chaucer. But most know very little about grammar, which is usually taught in linguistics departments, not English departments.

So when you combine the false confidence that comes from the title "English major" or "English teacher" with an utter lack of grammar training, it can lead to brazen errors like "my husband and I's first trip" where a more timid user might go for "my husband's and my first trip" or "my husband and my first trip."

If those "timid" choices sound better, it's because they are better. "I" is a subject pronoun. "I made lasagna." It can never be used as a possessor (more properly: a possessive determiner), as in "Bob loves I's lasagna."

So we can say with confidence that Englishteacher702's choice was wrong. But what's right? Well, that's not as easy to answer.

The major style guides contain only limited discussion on what they call "joint possessives" or "shared possessives." According to the Chicago Manual of Style, the Associated Press Stylebook, Words Into Type and Garner's Modern American Usage, when two or more people possess something jointly, only the last one gets a possessive marker — that is, an apostrophe and an "s." John and Mary's house. Pete and Sue's vacation. Mom and Dad's marriage.

But when the nouns possess things separately, each gets its own apostrophe and "s." John's and Mary's shoes. Pete's and Sue's jobs. Mom's and Dad's opinions.

Great stuff, right? Clear, simple, easy. But there's a problem. The style authorities don't tell you what to do when one of those names is swapped out for a pronoun form like my, his or your. Is it "John and my house"? Or "John's and my house"? They're not saying.

Usually, when a style guide fails to address a specific structure, you can assume that the broader rule applies. So you might guess that when you take "Mary's" out of "John and Mary's house" and replace it with the possessive "my," you would leave John alone: "John and my house."

That, according to a strict reading of the rules, might be correct. But I'm not buying it. Even when possession is shared, "John's and my house" sounds better to me than "John and my house."

I was curious whether it was just me. So I surveyed four editor friends. I made them choose between "This is my husband and my vacation" and "This is my husband's and my vacation." All four preferred "my husband's and my." And that's my (not I's) preference too.

JUNE CASAGRANDE is the author of "It Was the Best of Sentences, It Was the Worst of Sentences." She can be reached at JuneTCN@aol.com.