The problem is it assumes the valid indices are 1:length(a). This cannot be assumed for an arbitrary AbstractArray:
julia> using OffsetArrays
julia> A = OffsetArray(1:10, -2)
1:10 with indices -1:8
julia> A[length(A)]
ERROR: BoundsError: attempt to access 10-element OffsetArray(::UnitRange{Int64}, -1:8) with eltype Int64 with indices -1:8 at index [10]
Stacktrace:
[1] throw_boundserror(A::OffsetVector{Int64, UnitRange{Int64}}, I::Tuple{Int64})
@ Base .\abstractarray.jl:691
[2] checkbounds
@ .\abstractarray.jl:656 [inlined]
[3] getindex(A::OffsetVector{Int64, UnitRange{Int64}}, i::Int64)
@ OffsetArrays C:\Users\kittisopikulm\.julia\packages\OffsetArrays\N7ji6\src\OffsetArrays.jl:428
[4] top-level scope
@ REPL[100]:1