The most time-wasting bug I've ever had in a project of my own was in
the printf() code for PDCLib. I stared at it, I tried to trace it, I step-debugged it, and this cockroach just avoided "being seen". In the end it turned out that I was adding to the wrong variable, leading to not-all-the-time off-by-one errors in space padding.
Funny enough, I
found the bugger during the Breakpoint 2006 demo party (hence the dedication in the source comment). Couldn't find it sober for weeks. But after three days of beer and junk food, in a packed and noisy and smelly coder party hall, it jumped at me and said "here I am, you twerp".
As for time-wasting
projects... as I don't consider PDCLib a wasted effort in the least, I don't count the hours spend on it. I am
not so sure about one other non-computing project. I re-edited the rules for
Rolemaster 2nd Ed. and translated them to German. For copyright reasons, I will never be able to distribute this work to any extend -- "fair use" allows me to pass out copies of it to my players, but that's it.
588 pages of table-rich 10pt two-column roleplaying goodnees, layed out in LaTeX glory. Took me about five years, working on it basically all my spare time.
The former C.E.O. of Iron Crown Enterprises, Coleman Charlton, called me "a little nuts" for doing this.
I have yet to actually start a group, because "life happened" between completion of the tome and now. But things are looking better these days.