Here are some of the things that first-time occam programmers in the CO516 classes I taught in 2004 found confusing.
Week 2: bugs.occ
The IF syntax; most of them tried "IF condition".
Variable declarations before the blocks that use them (figuring out where to add a SEQ).
Indenting (and "outdenting") large blocks of code; they found it frustrating until shown how to get VIM to do it automatically.
Warnings from the compiler about unused top-level channels.
Lack of implicit SEQ. (They all understood this when it was explained, but a couple of them rolled their eyes...)
Types in PROC arguments applying to multiple parameters.
(Thanks to Matt Jadud for these.)
Colons at ends of PROCs.
The intersection of PAR and SEQ. For example, being able to recognize that
PAR processA processB PAR processC processD
is equivalent to
PAR processA processB processC processD
PAR processA processB SEQ processC processD
means that you have three "threads" of control, and in the third, two things are happening in sequence in parallel with the first two threads.