For my $.02 worth, first I have no problem with MWBCs, I can see the problems with an open neutral or an overloaded neutral, which would usually come from improper installation. I personally like them for a few reasons; first, less wire to pull and second, a less crowded breaker panel (half the neutrals and grounds). I guess I like them since I have yet to see the horrors of a MWBC gone bad.
Second, I have seen something funky like that, but it wasn't because of a bad neutral. It was on a house (my grandmother's...I've been hounding her to get her service upgraded) with a 60A service with a cartridge fuse disconnect (I fused it to 55 A/leg). Anyways, there was alot of cooking, etc, and one of the fuses blew. The strange (at the time, I know what happened) part was the lights on that leg didn't go out, they just dimmed, and the microwave even made an attempt to work. Since the oven was on, the blown side was putting its hot thru the heating element of the stove (hence the dimming) I figured it out quickly after turning the oven off and the lights went completely out. Anyways, it was strange, and strange to see the current cross over a 240V load to get to the other leg.
