Descriptive Problem Summary:

Essentially, object icons are being replaced by other icons (that don't even need to be loaded on another instantiated object, icons that have never been used ingame,) randomly. This tends to happen to objects with a lot of appearance churn.

This started occurring around late December. We switched from 512.1484 -> 513.1500 -> 513.1502 around that time.

We think this is due to a large amount of appearances/images being sent to clients, and it glitching out somehow. Perhaps having a large number of unique appearances in world and telling the client to dynamically load/unload more than it can handle. Some kind of swapped reference?

We don't think it's due to a packet issue, since we tested tampering, duplicating, throttling, and out-of-ordering all traffic.

We were able to cut down on the frequency of this behaviour by lessening the number of images we were drawing to the client (all mobs were drawing invisible (alpha 0) images to their screen on every single tile).

This issue is completely client-side, as if you reconnect the corruption is no longer present.

Perhaps this could've been caused by the icon GC crash fixes earlier in 513?

Numbered Steps to Reproduce Problem:

Tried for multiple hours to get a test case working. Couldn't.

Expected Results:

No corruption

Actual Results:

Most frequently happening to all of our lighting objects which are on a separate plane with BLEND_MULTIPLY. Examples:
https://cdn.discordapp.com/attachments/383743035894267905/ 697701520665214996/Goonstation_Mass_Construct_IV_2020-04-09_ 015621.png
https://cdn.discordapp.com/attachments/383743035894267905/ 667805594677411853/dreamseeker_TvNOMP51kE.png
https://cdn.discordapp.com/attachments/383743035894267905/ 690638976138215454/unknown.png
https://cdn.discordapp.com/attachments/383743035894267905/ 676641565300752432/7425cdbe9d6f81c904cd8491e3e1d098.png
https://i.redd.it/pttvpju5vkv41.jpg

Here's it corrupting to a really huge object:
https://cdn.discordapp.com/attachments/544105760167034890/ 704691129089523762/unknown.png

Player sprites corrupting:
https://i.imgur.com/Gf7Uvfs.png

Does the problem occur:
Every time? Or how often? Infrequently.
In other games? Unknown.
In other user accounts? Yes.
On other computers? Yes.

Did the problem NOT occur in any earlier versions? If so, what was the last version that worked? (Visit http://www.byond.com/download/build to download old versions for testing.) This didn't occur before 513.1500 to my knowledge. We updated from 1500 -> 1502 around when this began to be noticed.

This still occurs in 513.1520.

Workarounds:

Reconnect, and it will sort itself out.