Some vehicles' magazines[] values within CfgVehicles have a (lower/upper) case that differs from the associated CfgMagazines entries. This usually goes unnoticed, since the '>>' operator is not case-sensitive, but checking array-membership via 'in' will fail.
Example (notice the "belt"/"Belt"):
CfgVehicles/MBT_01_base_F/Turrets/MainTurret/magazines[] = { "2000Rnd_65x39_belt", ... }
vs.
CfgMagazines/2000Rnd_65x39_Belt
Without this knowledge this takes a significant time to debug since 'in' fails for no apparent reason.
How to fix: Use case from CfgMagazines for CfgVehicles/**/magazines[]. {F25315}