Textures suddenly start changing randomly in-game!

Think also about the default animations! For example, in vanilla, you have the animation FIREWALA, FIREWALB, FIREWALL. If the textures aren't in this order, then they will cycle using the long way around. Suppose you have FIREWALL, FIREWALA, FIREWALB. Now, the animation is defined as being "every texture between FIREWALL (last) and FIREWALA (first)", but the direction in which they cycle is always the same. So if you put the last before the first, then the animation cycle will include every single texture in the game except the one in between the last and first, because it'll cycle the long way around, wrapping around once it reaches the end of the texture list.

Here's an illustration using the alphabet. Suppose you want the substring that starts with M and ends with O. You'd expect to get MNO. But our alphabet has been jumbled a bit and is not in the regular order:

ABCDEFGHIJKLONMPQRSTUVWXYZ.

So you start at M, which is incorrectly located, and keep adding letters until you reach O. You'll need to wrap around to A once you reach Z. So instead of MNO, or of ONM given the jumble, you get MOPQRSTUVWXYZABCDEFGHIJKLO.

This is basically what's happening here. And vanilla animations affect all textures that are between the start and end of the animation, so again to use our comparison, if you look at W or F, these letters are "between" M and O because of the mistaken order (but N isn't).

Sounds to me like there's an issue in your ANIMDEFS lump. Try backing up then deleting the ANIMDEFS lump, see if the random changing gets fixed. If it does, re-include the ANIMDEFS lump and go through checking each of your animations ensuring it looks correct.

If it's not ANIMDEFS then I have no idea, heh.

Good luck!

Thanks Dragonfly, I'll give it a shot. 

SOLVED!

@Gez You are correct : ) 

After reading your post I messed around trying this and that for a while but with no luck so eventually i started from scratch and re-extracted all the flats / textures in Slade now it's all back to normal. Thank you so much for your help. 

This issue is basically my fault as it's connected to an different problem i had with textures changing colour. Different textures with the same name but different texture packs were getting mismatched in GZDB. Eg: WFall1 (Jimmytex) kept getting automatically replaced by WFall1 (ultratex). This was happening even though I'd separated all the flats / textures into their separate folders in the pk3. So i went and erased some of them and renamed a few others to fix this problem which worked initially but after a while my map went nuts. So now there's no blinking random texture changes, I guess a better solution would be to rename the offending textures something that's still in numerical sequence eg. WFall1.5 (Ultratex). Hopefully that'll work. 

Update: seems to be working fine : )

2nd Update: For anyone who encounters this problem, renaming the textures actually DOESN'T solve it because the bug came back somehow. I think this bug begins when you load a saved file. Better off just deleting the offending textures. I've done that and now it's ok.