Trap representations and padding bits
The C programming language does not hide from you how the values you manipulate are represented. One consequence is that when padding happens, its presence may have observable effects in carelessly crafted programs. Padding is well-known to appear between members of a struct, and also possibly after the last member of a struct. The remaining space in a union when the active member is not the widest one is also considered padding. A C programmer that only cares for usual x86 platforms might be excused for thinking that, for them, this is it. As for trap representations, these may be believed to be reserved for weird hardware that use one’s complement or explicit parity bits.
The example at the end regarding _Bool is nice.