The flight took off from San Francisco Friday afternoon and was ultimately on its way to Denver -- but it was forced to divert to an Oregon airport in between ... all because a panel near the bottom of the plane got torn off mid-flight, exposing the underbelly of the aircraft.

The photo of the damage -- taken after the 737 plane successfully landed -- is startling ... you can see the guts of the plane inside ... with a bunch of plane parts fully exposed.

You can also see where the panel itself broke off ... there's a clear tear in the material -- which, again, is alarming, to say the least ... especially with everything that's been going on with Boeing lately, and there's been a lot.

While reports say all passengers in this incident were safe -- it, unfortunately, marks the latest Boeing mishap to occur in a relatively short window. Remember, Boeing planes -- some of which have been on UA flights -- seem to be falling apart all over the world.

One plane lost a wheel last week, and another Boeing plane had flames shooting out of one of the turbine engines while up in the sky ... and had to make an emergency landing.

Of course, there's also the LATAM Boeing flight in Australia that briefly started to fall out of the air ... although, now, they seem to be suggesting that was user error.

That doesn't make things much better though ... especially after a door blew off earlier this year on yet another Boeing plane. And let's not forget ... there's still this whole whistleblower saga going on.

Remember, John Barnett died this past week while in the middle of a retaliation lawsuit against Boeing -- this after he alleged they'd been cutting corners on safety measures for years. Cops say it appears he took his own life ... but his attorneys have been calling BS.

Bottom line ... it's choppy waters for Boeing these days.

As for today, UA says ... "We’ll conduct a thorough examination of the plane and perform all the needed repairs before it returns to service. We’ll also conduct an investigation to better understand how this damage occurred." The FAA says it'll investigate too.