When adults act like kids

One of my co-workers is turning 40 this weekend. She posted an open invitation in our staff room, so everyone knows that they're invited.
One of my other co-workers is celebrating her 5th year wedding anniversary this weekend. It's their tradition that they invite a bunch of their close friends to a BBQ, and have a friendly game of baseball.
The lady who's having her birthday party found out about this "other" party. She approached the other woman, and asked her to re-schedule her party because that was the day of her party....
The other woman was quick to respond that her get together was at noon and was expected to be done by 4 p.m. Suddenly, that was acceptable, and all is right with the world again.

The same woman who's having her 40th birthday this week was also mad at me.
One week before my birthday, I invited a few friends to join Ali and I, and go 5 pin bowling.
One of the girls that came out with us invited everyone back to her place afterwards for a few drinks. We declined, and headed home - unaware of the drama my actions were about to cause.
The next day, some of the people were talking about the bowling, and the shenanigans that followed at the "after party".
This woman overheard their conversation, and snipped "Chris never told me about this party. How come I wasn't invited?".
My first clue is, if somebody doesn't ask you to join them, you're probably not wanted. I guess that's just me...
Her next comment was priceless. She said "I WAS going to throw Chris a baby shower, but now he can forget it!".
What are we, eight?

So, my advise to everyone is to post a general invite, extended to all, even if you don't want everyone there. If you don't like the company, you can sneak out the back door.
There's never a shortage of drama at my workplace. It's a real-life soap opera, but with uglier people.