I've encountered a bug involving one specific sprite set, pulled from the base files. Error message shown in the image link below.
This applies to all available versions.

https://puu.sh/G9BlC/dce1d620b9.png

My workaround:

[Forgive my lack of perfect lingo; I'm not a game dev or even VBA guru]

Evidently the sprite coordinates it's trying to pull are usable when using the base mod's high-res file (path below), but it's trying to use the low-res version which does not have wide enough boundaries for those coordinates.
My fix simply involved opening the low-res version in Paint, expanding the boundaries to match the high-res one (210x480, but 220x480 also apparently works), and stretching the original area (110x120) to fit.
I have not yet tested what visual effect this has in the game.

Affected file path: \Program Files (x86)\Steam\steamapps\common\Factorio\data\base\graphics\entity\slowdown-sticker

I can only assume that an update to the base mod, Rampant, or the base game may revert this change and return the error.
If anyone has a more permanent solution, please advise.

(this mod is tied for my favorite content changer in the entire mod library and I can no longer go back to playing without it. Thanks to everyone involved!)

I am also getting this same error.

Having the same issue

I am also having this same error, however it only started AFTER I enabled new enemies in mod settings. I could not disable this setting because the setting remains ON even after you remove and reinstall mod.

I found two workarounds.

The first is deleting rampant mod (trash icon in mod list), then deleting mod-settings.dat in the AppData\Roaming\Factorio\mods folder, then reinstalling rampant and LEAVING NEW ENEMIES disabled.

Alternative that gets rid of the error and allows new enemies settings but might break things:
go to C:\Program Files (x86)\Steam\steamapps\common\Factorio\data\base\graphics\entity\slowdown-sticker
rename slowdown-sticker.png to something else (slowdown-sticker.png.backup), then copy hr-slowdown-sticker.png and rename duplicate as slowdown-sticker.png. This made the error go away and allow new enemies setting, however I have not yet tested it yet.

Worked perfectly. Thanks wolfmyst!

Hi everyone!

Maybe the better solution will be to replace the line used in Rampant mode.
The file is here: Rampant_0.18.16\prototypes\SwarmUtils.lua
Here is the line: filename = "base/graphics/entity/slowdown-sticker/slowdown-sticker.png"

If replace it by filename = "base/graphics/entity/slowdown-sticker/hr-slowdown-sticker.png", no error occur. Will test how it works in game.

Having same issue

Both wolfmyst and mitasamodel's solutions work; I was able to load into the game and load a save successfully for both changes individually.

I suspect that mitasamodel's solution is indeed better, as you wont have to re-apply the change with every main game update.

For those unfamiliar with where to find your mods folder, it's at C:\Users\<name>\AppData\Roaming\Factorio\mods
You will have to make the change while your game is not running, or it will not save properly.

Works for me, but I found slowdown sticker line in
filename = "base/graphics/entity/slowdown-sticker/slowdown-sticker.png",

I've made a small mod that applies these fixes without having to manually edit the original Rampant files, mostly useful for automatic mod syncing in multiplayer.

Here's the link for anyone interested: https://mods.factorio.com/mod/Rampans

should be fixed in the latest.