Algodoo

For most creators this bug is very difficult to notice since it requires a lot of time and the use of grid aligning for it to be noticeable. I have read about it being documented at least once by someone on an algodoo forum.

Ozziegerff -- I think I know what's causing the bug that you described but I cannot be certain unless you post a scene which demonstrates what happens. Just your verbal description is not enough because there may be things going on that you do not realize. Does the bug show up in the pinball machine that you included in this scene? If so, explain in detail what happens.

It happens in all scenes, randomly starts and stops. It seems to be tied to objects that are the same as it (copy and pasted or cloned ones) and behaves very strangely, here is a video of it happening.
https://www.reddit.com/r/Algodoo/comments/smgd6b/circle_drift_glitch/

Okay, yes I have seen that before, and here is a workaround that seems to work for me. First of all, as you stated, it only seems to affect circles (not boxes) that are glued to the background. When I create a bunch of circles by using the snap-to-grid feature, each circle appears to be exactly the same size as the other ones, and they appear to be in very specific x,y locations. When all of the created circles are glued to the background, a very odd behavior occasionally occurs. For example, if I monitor the pos value of any one circle, and then I slightly move a different circle, the pos of the monitored circle will move a small amount! It doesn't always happen, though. I can often nudge it to happen by first pushing around the circle that I'm monitoring, and then I put pressure in the x or y direction on some other circle with the grab tool, the pos on the circle I am monitoring will sometimes change slightly. Sometimes it will happen and sometimes it won't. Sometimes it will happen for a few seconds and then quit! Oh, and this odd behavior works while Algodoo is paused or running.

Here is my workaround: If you have a large number of circles to make which must be exactly the same size as each other, and they must not move from where you put them, CLONE each one from a single circle rather than to create each one individually. It's as simple as that! Here is why I think that works: When you create a bunch of circles individually, even when you snap to grid, the size of each circle may be a very tiny fraction of a mm different from each other. The difference will not immediately show if you look at each circle's pos, but does occasionally show when you move any of the glued circles. I don't know WHY that happens. It's bug that will probably never be fixed, and so if my workaround works for you, then problem solved. If it doesn't work for some reason, then I don't have any other suggestions at this point.

Just an update.... I tried my workaround on my wife's computer (I had to download Algodoo because, sadly, she has no interest in it). I found that this weird circle bug is computer dependent. What I mean is, the bug happens quite regularly on her computer even with my workaround, but it does not not happen on mine! WEIRD.

did she have an apple, Im on a 2013 macbook air running osx sierra?

No, both of our computers run Windows-10, but mine has about double the RAM that her's has. I don't know if that's what makes a difference. Oh, and her's is a laptop and mine is a tower type desktop.

I see

Yeah, I know... So irritating. The funny thing is that it applies to all objects instead of just circles. I'm running 2.1.3.

I had this bug in my videos so I added this kind of script: I added the position of the object and made it into a variable. In postStep, I added (e)=>{pos = _pos} so then the position wouldn't change no matter where you drag it. You could change it though. But mostly I wouldn't mind about it. I hope this helps. and yes, it does apply to any object like how my gadget bus keeps drifting.

MinecraftJa123A -- There's actually a simpler way to achieve the same result. Put curly brackets around the pos vectors like this:

{
[4.8772764, 4.3252554]
}

When you do that, it freezes the vectors. The only way you can move the object is to delete those curly brackets manually, or you can do it with scripting such as:

pos = pos + [0.1, 0.0]

or even simpler: pos = []

it seems to only happen when i have a lot of objects in one scene for me.