Goofs

This movie picks up the action from Flash Gordon (1936) as they return to Earth from Mongo (ie: They are still returning from Mongo). At the end of that first movie, they took off for Earth in Zarkov's Space ship but they are now in a Mango Space Ship (with the nose ray gun). The cliffhanger of Chapter 1 has one of the engines shot off the ship and at the start of Chapter 2 (after the cliffhanger resolution) Ming looks at the engine and says "It is from the ship they stole from me".

Although filmed 2 years later, this story takes up exactly where the original ended. Dale Arden was a blonde when she entered the ship, but somewhere in mid-flight, she changed to a brunette. Even if filmmakers/studio heads were counting on audience short-term memory, a flashback sequence is there to remind us Dale was originally a blonde.

It's such common knowledge that it hardly bears mentioning, but of course Mars's comparatively thin atmosphere lacks the oxygen necessary to support complex life as we know it, so the entire scenario of humans and Mongonians roaming around on the surface without space suits is purely fantastical, and indicative of the hypothetical nature of the planet's on screen counterpart.

In one scene where Queen Azura vanishes, you can see her run before the flash powder used for the effect ignites.

In Chapter: 12; after Flash rescues Dale, while Flash is talking to Azura a fly lands on Flash's arm. Flys couldn't survive on Mars.

In a couple of scenes we hear Dale scream, but we don't see her mouth open.

When Queen Azura's death squad is ambushed by the Clay people and their nitron guns are stolen, even if you accept that the Clay people can walk through the cave walls because they are made of clay, the nitron guns are solid and would not be able to go through the walls.

Near the end of Chapter: 8; when Ming throws the switch to trap Flash, there are neon signs mounted sideways on the control panel. One says "M-A-D-E". The other is two neon signs back to back. You can make out the letters "E-P-I-N-E-S. Right before this, the boom mic shadow is visible above Flash.