Okay the title is a little misleading. It's not so much that the combo didn't work as it either didn't work as well as you hoped, or required just too much investment for what was essentially a gimmick. I have two:
1) French kissing a Sea Urchin: The title references an old Silent Death joke. Anyway, this was an Assault Frigate Mk2 A with Spinal Armaments and Paragon, giving it a four dice arc in all four arcs and an extra die if you got double arc'd...how could that not be awesome? Approaching such a ship would be like trying to french kiss a sea urchin...in theory. In practice those points seem to be better invested in a Gunnery Team and reliably keeping 2+ enemy ships/squadrons in your side arcs. It's not useless, it just wasn't quite as cool as I was hoping.
2) Junk Knight: This one seemed obvious to me. A YT-1300 freighter with Jan Ors flying escort. At a wopping seven hull, with escort and the ability to use Jan's Brace tokens, how could this not be great? It could soak up damage for days and protect your more fragile ships. The reality...well I found it fell short in two places. First the speed...speed 2 is way too slow for an escort, it often cannot keep up with the ships it wants to protect. This can be somewhat mitigated by the Independance title or the new Flight Officer, but then you're sinking still more points into a mediocre combo. Two was a little thornier as I suspect it comes more to opinion but...I don't find Rebel Squadrons really need escort all that much. Most boast at least five hull and A-Wings would be way too tied down hiding behind a lumbering two speed guy. HWK-290's do benefit some from that protection, but I'm not sold on fielding one just to protect HWKs.
So those were mine. Any combos you guys imagined in your heads that didn't work out so well on the table?