Lately my shower has been acting kind of wonky, supplying only really hot or really cold water depending on how I fiddle the knob. Even more recently, the knob up and fell off, and irreparably chipped the plastic holding it onto the rotating protrusion sticking out of the wall. So I call the landlady's fix-it dude over and ask him whether the handle can be replaced in isolation; he says no, they're usually proprietary and you have to replace the whole "diverter". That is, the entire piece of machinery that contains the handle and pipes and so on that allows diverting (ideally) continuously varying amounts of hot and cold water into the shower. He drives over to home depot to get a new one and the appropriate miscellaneous copper pipe bits, and comes back and solders them all together.
Here depicted is the old one, before he went to town on it with some pipe cutters and installed valves and what-not. _tove was around and talked with him (far more knowledgeably than me obviously) about the finer points of soldering.
Conclusion: The new diverter fixed both the "have to bring a pair of pliers into the shower to use it at all" problem and also the "binary water temperature" problem. -10 modularity points for not being able to just replace the handle, but +10 modularity points for lovely little bits of copper pipe, arbitrarily solderable to one another.
The other problem was that the back of the toilet was leaking water, for unknown reasons at first. The site of the leak was a little white plastic widget, (hereafter LWPW) which I tried loosening a little. Heard a CRACK, faint but clearly of great significance, and water started spurting out more enthusiastically. Turned the valve off. Wondered how the hell to get the (now clearly cracked, where once it presumably had a small but still leak-causing defect) LWPW off.
_tove tried epoxying over the hole, which seemed like it was going to work, but didn't; maybe we just didn't wait long enough for it to set. Now that I read the back of the package it says "sets in 5 min, full strength in 24 hours". But no matter: it seemed easy enough to just go to Home Depot and buy another LWPW, whatever it was properly called.
Trouble was, even after I completely removed the segment of pipe connecting the water supply to the toilet, the LWPW still seemed to be firmly set on staying on the pipe. Its largest hole was still too small to cajole it off the smaller end of the connector.
So I went to Home Depot sort of dejectedly, pipe thingy in hand, expecting the worst, that perhaps even the hex-nut end of the connector required some kind of adhesive and not just friction, but left with the pleasant discovery that not only was this not the case, but also that what they had for sale (for just $4) was the entire apparatus of (flexible) pipe plus hex nut plus LWPW.
Conclusion: Installed it with little fuss, and now my toilet works great. -10 modularity points for making it difficult or impossible to replace just the LWPW, but +10 modularity points for inventing flexible pipe segments so I didn't have to worry about getting the exact right length.
Maybe after all is said and done neither of these stories features unmodular design - it's just that the abstraction boundaries were higher up than I expected.
Thanks to _tove for helping out, and escargonaut for telling me that there was a Home Depot in East Liberty on the 500 route!