ZDoom

I dunno but this commit looks relevant:
https://github.com/coelckers/gzdoom/commit/b2ccd0bd28475793b8f16fb3324ed393bdef5e9a

Setting the last parameter to 1 will not increment the counter. This was definitely never meant to be used like here but it should work anyway.

I don't think you can have several calls to A_CheckForReload inside the same loop, after all it's a generalization for A_CheckRailReload. May be before it happened to work "as intended". I know that jumps are handled differently now but the code for that function is pretty much the same as before.

Okay true but he's changing the counter in the middle of the loop is that right?

This has always been possible in DECORATE. The documentation has it wrong.

Are you sure? It's not documented and it is in A_Jump.

Integers are accepted as an offset just like pretty much all other A_Jump functions.

Well according to the A_CheckForReload spec i can't make head and tails of your example. The second parameter is a string not an integer and you pass always the same (besides it doesn't exist).
https://zdoom.org/wiki/A_CheckForReload

Attached to OP

Post a runnable sample please.

ChooseFireFrame:
    QSNG # 0 A_CheckForReload(8,1)
    Goto FireFrame7
    QSNG # 0 A_CheckForReload(7,1,1)
    Goto FireFrame6
    QSNG # 0 A_CheckForReload(6,1,1)
    Goto FireFrame5
    QSNG # 0 A_CheckForReload(5,1,1)
    Goto FireFrame4
    QSNG # 0 A_CheckForReload(4,1,1)
    Goto FireFrame3
    QSNG # 0 A_CheckForReload(3,1,1)
    Goto FireFrame2
    QSNG # 0 A_CheckForReload(2,1,1)
    Goto FireFrame1
    QSNG # 0 A_ResetReloadCounter
    Goto FireFrame0

Seems A_CheckForReload is a little bit broken in GZDoom under certain circumstances.
I have a weapon that has sequential fire frames but may be canceled at any time.
Due to Zandronum compatibility (hurr durr I know...) my choices are limited and if I don't want the weapon animations to be ping delayed I can't use ACS calls.
I choose the next frame by using this:
[code]ChooseFireFrame:
QSNG # 0 A_CheckForReload(8,1)
Goto FireFrame7
QSNG # 0 A_CheckForReload(7,1,1)
Goto FireFrame6
QSNG # 0 A_CheckForReload(6,1,1)
Goto FireFrame5
QSNG # 0 A_CheckForReload(5,1,1)
Goto FireFrame4
QSNG # 0 A_CheckForReload(4,1,1)
Goto FireFrame3
QSNG # 0 A_CheckForReload(3,1,1)
Goto FireFrame2
QSNG # 0 A_CheckForReload(2,1,1)
Goto FireFrame1
QSNG # 0 A_ResetReloadCounter
Goto FireFrame0
[/code]
In Zandronum and GZDoom versions up to 2.1.0, the frames go 0, 1, 2, 3, 4, 5, 6, 7 as they should, but in GZDoom 2.2.0 and later it goes 1, 2, 3, 4, 5, 6, 3, 7 which isn't right. Note it starting at 1 instead of 0 and the 2nd 3 before the 7.

I've spoken to phantombeta on Discord about it and after a bit of back and fourth, they said:
[quote="phantombeta"]
Congrats, either you've found a very old bug that no one else has been affected by, or your code was accidentally relying on undefined behaviour
You should report it on the forums now
[/quote]