For me, it's fenders.
I've never had a clean fender installation. My first set of fenders was kludged onto my old Nishiki Custom Sport using bolts that were definitely not meant for a bicycle, some gorilla glue, and some spare deraileur hangers (please don't try this at home). Even though I bought them under the label of being 27" wheel fenders, in no way, shape, or form did they fit my 27" wheel.
My second install went a little better - I was putting some simple carbon fiber fenders on my road bike, which I've showcased here before. This did require me finding some things, like Sheldon's problem solvers, a dremmel, and sticking with tradition, more gorilla glue. And it was in no way because I was lacking the hardware needed for installation - the bike has eyelets, the fenders were the right size, but somehow adding me into the mix created this terrible combination that requires thinking slightly outside the box.
It's because of this that I decided to never touch fenders ever again, but then I remembered that A) I now live in the bay area and B) it rains a lot here. So, I bit the bullet, and decided to try again with putting some fenders on the bike I'm currently using a winter/train/hope-on-while-wearing-jeans ride.
I decided this time, if I needed a tool for this job that I didn't have, I was going to invest in it no matter the cost. No longer am I a broke student trying my best to fix bikes with a mixmatch of wrenches collected from various sets of Ikea furniture, I'm an adult, and dammit, I want a nice tool collection.
Keeping with tradition, the set of fenders that I bought required Problem Solvers, which makes zero sense to me as it's 2012 and to my knowledge a good majority of bikes are now made with recessed brake bolts, but hey, maybe I'm wrong. And despite being for 700c wheels, the attachments were way too long, so I went and obtained bolt cutters instead of borrowing a dremmel.
Miraculously, after bolt cutting the front fender attachments, the front fender went on fine. With a renewed sense of hope, I moved onto the rear fender.
Since there was no way that this could have possibly been easy, the rear wheel didn't have the right size tire to fit with the fenders, as I was running a 700*28 in the rear. I pulled out a spare 700*25 that I had and swapped the tires, and then did some measuring. Once again, the attachments were too long, so they had to be cut. And then it seemed like every other part either required the wheel to be on the bike or off the bike, or simultaneously on and off the bike in order to properly measure things and install bolts.
Once the fender was mostly on and I still had a little bit of sanity left (about three hours later) I completely reinstalled the back wheel and tensioned the belt, to which I should mentioned that belt drives, in my opinion, are a piece of cake compared to fenders. I finally got the back fender on, then spent about an hour tweaking, repositioning, tweaking some more, and adjusting wheel clearance before I was sort of satisfied.
I've also come to the conclusion that someday, when I open my own shop, I will have to hire a fender specialist. I wouldn't wish my fender installations on my worst enemy.