I'm running mostly newer players through Rise of Tiamat, have 2 experienced players and 4 newer and younger players. They got to one of the cult encampments and try to sneak in by impersonating new recruits (this idea came from one of the newer players so was somewhat impressive). The one player panicked (failed persuasion checked role played the sequence) however he literally forgot where they were and has his character yell "PRAISE BUHAMET"....

I just face palm and start laughing, and pulling out all the minis. The experienced players both groan and face palm. The player that made the statement is literally trying to figure out what he did wrong. The other newer players were upset because the camp was charging them from all directions, and they weren't happy they were probably going to have to make new characters.

Of all the words in all the languages of DnD 5e that he could have said, he said the one word he shouldn't given where they were.

After 10 minutes of laughing and setting up the hoard about to crush the party, they ended up living by some creative actions by the experienced players along with favorable dice roles, I finally explained who Buhamet was in relation to Tiamat and he just looks at the table, the hoard, and the other players and says, "I am so sorry if we make it out of this alive you can kill me and I won't even be mad".

Anyway that's my recent epic fail story.