Dennis Quaid's most mortifying moment? Not Randy Quaid's burglary arrest.

Dennis Quaid's most embarrassing moment? 

Hint. Not the recent burglary arrest of Randy Quaid and his wife, Eva. Although you would think that would be it -- Randy and wifey were arrested yesterday and are being charged with skipping out on a $10,000 hotel bill at San Ysidro Ranch in Montecito. 

No, Dennis' most embarrassing moment was during his appearance on Ellen DeGeneres' show a few years ago.

Watch the video. It's self-explanatory. And hysterical. 

"Day Oh!" 

The checkout guy is so thinking, "What is this guy on?"

We give Dennis -- whose new sci-fi movie "Pandorum"  (watch the video here) is in theaters -- major props for doing this stunt.

As for his brother? Hope Dennis can persuade him to get help.

Everyone knows that only A- and B-list stars get free rooms. Yes, Randy won a Golden Globe, but that was a long time ago.