Fasm has no include cycle dectection, thus if such a cycle exists, you
will probably always run out of memory.

Unfortunately it is not possible to conditionally include files, since the
preprocessor doesn't evaluate such conditionals. So if you write
something like

_WINDOWS_INC_ equ 1

IF NOT DEFINED _WINDOWS_INC_
include "incfile.inc"
END IF

the preprocessor would include the file despite the false condition.
This seems to be a waste of memory and time since at the stage of
evaluation of this conditional expression, the evaluator has to find
the dangling END IF and therefore has to process the hole "incfile.inc"
content.

Some solutions were provided here.
Also please read the Design Principles article (section 4) to see why fasm's IF works this way. However I'm planning to add some IF-like directive processed by preprocessor instead of assembler in the near future.