PSD is not my favourite file format (2009)
&gt; Why, for instance, did it suddenly decide that <i>these</i> particular chunks should be aligned to four bytes, and that this alignement (sic) should <i>not</i> be included in the size?<p>Not saying this happened here, but I have seen this type of mistake before. It was because they &quot;simply&quot; cast a C&#x2F;C++ struct to a binary blob <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Data_structure_alignment#Typical_alignment_of_C_structs_on_x86" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Data_structure_alignment#Typic...</a> and wrote that to disk (or sent over the network in my case). So that particular compiler version and architecture-specific struct field alignment became the &quot;official&quot; format.  It just takes one goofy mistake like that and everyone has to deal with it for years to come.