Weird error message when I play-test my new WAD i'm making

When I try to playtest my test level on WadAuthor, I usually could, except after I changed a texture, I got this message:

"Z_CheckHeap: block size does not touch the next block".

What the heck does this mean?

It's a memory allocation-related error which you get if your player ever enters an unclosed sector, or if the sector contains sidedefs which have either not been assigned to any existing sector or refer to a bad sector (such as -1). This generates a consistency check failure which tells you that the address of a block of allocated memory, plus its allocated size, does not touch the next block of allocated memory. To fix this, just remove the bad sidedef or refer it to a valid sector.

Uh, I have tons of unclosed sectors in my WAD and it runs fine, even in Vanilla Doom.

Doom is fickle sometimes.

It's the bad sidedef references, not open sectors. Open sectors may cause weird rendering, but as far as I know, they do not usually crash the game.

I haven't experienced an open-sector Z_CheckHeap crash myself, but I remember first having that error many years ago and finding a FAQ which listed that as one of the causes. It could have been true in earlier versions of the game and fixed by v1.9.