Mjoll often goes on to kill my Housecarl Calder in Hjerim after I dropped a gem or two.

I've found that the only way to prevent this from happening is to Pacify one of the two. Even when the spell works off, both still respect each other again.

I thought I shared this as I assume this can annoy a lot of gamers aside from myself.

Eventually people become to high leveled for Calm spells. even Harmony.

What about not dropping anything?

Have your follower wait outside if you want to drop items to decorate your home, a simple fix. Pick them back up when you're done or ready to head out.

Fully perked, dual cast Pacify affects NPCs up to level 83. Very few people can resist it.

I didnt know that. I'll use it from now on. thanks

You mentioned Harmony. Its problem is that master spells don't benefit from 2.2 increase in power, so dual cast Pacify is actually more effective in any situation, even without additional perks (25 vs. 44 level). Makes you wonder if the devs really thought this system through.

As for the OP, well... I've got to second "don't drop anything" advice. On the bright side, at least you know why they're fighting. Iona used to attack me without any reason whatsoever; just her way of saying hello, I guess.

Skyrims Magic is Tragic I enjoy using it but is probably the worst in the series. my favourite thing about it is actually equiping it instead of like oblivion were it can be used even when the characters hands are holding other things.

he might be dropping items to decorate though.

Don't drop anything.

For unwanted items, just dump them in random barrels. NPC's won't detect them. For decor, just leave your follower elsewhere (elsweyr?)