This tutorial is based of Antonio Roberts Datamoshing Guide. Check it out: https://www.hellocatfood.com/datamoshing-using-avidemux-2-7-0/.
You can go in an remove I-Frames manually for a more nuanced effect, but it's a lot easier to save the file with as many of the I-Frames stripped out as possible. This guide will show you how to do that.
While you can edit video in Avidemux, it's probably easier to trim your video first in. This technique works best when there is a significant change between clips. Don't apply transitions like wipe or cross fade.
To load your clip, click the folder icon, or select File -> Open
Normally, video editing software will automatically insert I-Frames where needed, but in this steep we'll be telling Avidemux to make a few as possible.
In the Video Output section, in the first drop-down menu that says copy by default, select Mpeg4 AVC (x264). Next, click the Configure button below that drop-down menu.
In the x264 Configuration menu, select the Frame tab and make the following changes:
After setting these click OK.
Finally, click the Save icon or select File -> Save to save a copy of the modified file.
Avidemux will still insert a few I-Frames. We will need to remove these so that the decoding errors will occur.
Close Avidemux, and then reopen Avidemux and open the file you just created by clicking on the Folder Icon or selecting File -> Open.
There will be one I-Frame at the beginning. We need to leave this one or the video will not play at all, but we still need to find and remove the other I-Frames. They will usually be near a section where there is a jump to the next scene or a lot of motion. Use the arrow keys to advance the play head till you find an I-FRM in the Frame Type indicator near the bottom of the window.
To delete the I-Frame, first click Set Start Marker (it looks like an A in a red box). Next, advance the play head one frame and click Set End Marker (it looks like a B in a white box). You can now delete the frame by pressing the delete key or select Edit -> Delete.
You may get the following error when you delete the I-Frame. Go on and select Yes.
Once you are done deleting I-Frames, save the file using the Save icon or File->Save. When saving, you can go on and give the file a .MP4 extension if you like.
Your finished product should look something like this.
Remember: This works bet with two clips with very different colors and styles, and clips with contrasting movement. Do experiment! you'll find different types or speeds of movement will effect the video in very different ways.