It may be an option that was later removed; in these cases those who already had it can keep it, but if they try to alter any part of the costume or powers (and confirm the changes), they'd loose it. I'm still kicking myself for overriding my magic bolero + wings costume while customizing powers when that came out.
"My inner mind has become a reality-cracking overgod. He torments me! Help!"
It's a trick of the costume creator versus crab spiders. You may need to have an alternate build which is not a crab, but you can set up a costume without a crab backpack and add the trenchcoat. In the costume creator the costume will look like it doesn't have a backpack. Then, as soon as you exit the creator, the backpack 'magically' appears.
Outside of the temporary effect of this quirky little bug, it's never been possible to combine the Crab harness with any other back details.
There are no words for what this community, and the friends I have made here mean to me. Please know that I care for all of you, yes, even you. If you Twitter, I'm MrThan. If you're Unleashed, I'm dumps. I'll try and get registered on the Titan Forums as well. Peace, and thanks for the best nine years anyone could ever ask for.
I swear I've seen many crab spiders wearing trench coats, with the tail still attached.
Does anyone know how to do such a thing?