When I replaced the knackered bottom bracket on my fixed gear over Christmas with the year-worn one from my road bike (which, although not new, was a better quality one and decidedly less knackered) I obviously didn't put the cranks back on well enough; although I've done it many times before exactly the same and never had issues.
Unfortunately, by the time I realised it was loose I'd gone about five miles, including up a couple of hills, and I didn't have the right size tool with me so I then had to limp home, stopping constantly to finger tighten the bolt. Which meant the crank was toast by the time I got home (although I didn't know that straight away) and could tighten it properly. Initially I tightened it up as much as I could and it lasted a couple of days. Then it worked loose again and I spent a week or so where I'd tighten it up and some times it'd last a few miles, sometimes the entire ride in, but eventually it got to the point where it was coming loose every couple of miles; I'm slow enough as it is in winter, without that happening.
I do have a spare left crank from my road bike (yay!), but it's a different length (boo!).
So I was left with figuring out a fix that would last a month or two until I could replace the whole chain set (Which is well overdue replacement).
At first of all I thought my chances were as knackered as the crank (due to searching and finding comments such as the massive negativity in this forum thread; I also "like" the assumption that everyone can just go and buy replacements for broken bits).
Fortunately I'm not the only person in permanent penury and others have been there and figured out a fix.
As per the linked post, it's two weeks also for me (actually a bit more as it's so hard to find enough time to write even an insignificant blog post like this). I managed to squeeze in four shims on mine. I did check every day first of all and with Herculean effort I could get an extra nip on it. Now I don't think I can tighten it any more. It's just got to last me another month or so...