Not so much a "Gah" as a "Meh..."
So I'm known as someone who can usually fix nearly anything, and usually for free / cheap due to my extensive junk pile of spare parts. As such I often get requests from friends and family to fix various household items etc.
Currently I am fixing a dead Dyson DC07 Vacuum cleaner for a friend of my mother, and it's one I fixed about two years ago - a simple fix due to worn out carbon brushes on the motor. At the time, I did warn that the only spares I had to hand were already well worn, and that it was not really worth spending money on to fix it further as the motor was quite worn due to age. However, it is back and sure enough it's the same issue - the brushes wore out, and this was accelerated wear due to the poor state of the motor commutator bars (badly pitted and small blobs of deposited molten copper due to heavy arcing because of the previous poor electrical contact).
Now then, a couple of years ago they paid for a dyson engineer to refurbish the cleaner, and it is in very good condition - but the motor was not part of the refurb, and this one is now pretty much done. BUT they would like to keep the cleaner if possible as it will save them some money, rather than buy a new one.
So here is the rub - the motor has worn bearings, but they are not excessively noisy, it's filled with carbon powder (which can be cleaned, mostly), and the commutator needs to be resurfaced - which is something I can bodge well enough to work. A set of new brushes is about £10 to £15, which is not bad, but the motor is very likely to fail LOOOOONG before those brushes do. At best I would say it has maybe another four years of life before the front bearing shits the bed.

It's not really worth spending money on new brushes, but then again, they are not
that expensive...
A new motor is about £50 (they will want a genuine one, not a cheap clone), but I don't think they want to spend that, and it's probably overkill for such an old model vacuum cleaner.
I do have a similar motor that I can take the brushes from, but they are currently in an unknown state of wear - and currently this will involve tearing down another Dyson cleaner, which is a fair bit of effort to waste if the brushes are already toast (I found it in a dumpster and took it for parts...).
If I wait for parts, it means I will have to leave a fully dissassembled Dyson cleaner all over my floor until they arrive and I can fit them...
UUUrrrrrrrgghhhh!! 
*Edit*
I stripped the other dyson down, found the motor was incompatible (there are two types fitted to DC04 vacuums), however it had perfect carbon brushes with minimal wear - suspect the motor was replaced only a few years ago. So I replaced the carbons in the original brush holders for the DC07 motor - which they are not designed to do - and luckily the carbons were a perfect fit.
It took a LOOOONG time to do, I only finished assembling the motor ten minutes ago, and now I have to rebuild the vacuum cleaner.
It seems I have killed a perfectly good working motor, and rebuilt a crappy one with bad bearings that will not last more than a few years at best. I now consider this a full
GARRRGH moment!
