What are some common mistakes students make with particle physics?
Wow! How long do you have?? It can be one of the most impenetrable subjects, but a good clear grounding can be achieved with careful instruction.
In my experience the single biggest barrier to learning is the plethora of words. Nearly all of them end in the suffix "-on" and students get very confused, especially when starting out. I recommend a family tree of the words, before you teach the details which you (and the students) refer back to several times a week until they are confident.
Understanding particle accelerators is another minefield which needs slow and careful exposition.
Students frequently get hung up on Feynman diagrams too.
Finally, the whole quantum theory weirdness smears a layer of Alice in Wonderland misunderstanding over the whole topic....
p.s. This remains one of my favourite things to teach!