Well, you never know when those kinds of things can sneak up on you. Once my chariot was in for repair and I wanted to go down to a little strip outside Philly (New Garden, N57) to pick up my daughter for the weekend. My FBO at HPN asked me to take an Arrow off the line that had a suspect DG on the squawk sheet to confirm the observation of the prior pilot(s). This was not unusual... they gave me a discounted rate in exchange for my opinions or diagnostics, though of course I declined any such offers if they related to the power plant, such as "low oil pressure", "prop regulator hunting", "misfiring at cruise" or "engine cuts out during climb", etc.
Anyway, it was a beautiful day and as three out of four of my hours were generally at night I decided to swing down past Lady Liberty for the first time in years and then dog-leg over, skirting Maguire AFB and resuming course. It was around noon with the sun dead above and when I got down around Old Bridge in the course of a few minutes the forward haze became MVFR at best, right about the time I needed to get back on the gauges due to less familiar ground references, make that right turn to avoid McGuire and yeah, I needed to check out that DG just in case I decided to fly back later that night. All strictly routine.
So I dialed in Yardley and Solberg, got a fix, threw in a few degrees WCA and set up my track to the west. I kept cross-checking that DG with the compass and sure enough it appeared whackey. Good vacuum, horizon behaving, just gradually precessing or something. Fine, on with the eye patch and follow the compass. Problem is, I still couldn't maintain a track, and without any outside horizon or sun angle there was no directional reference, either. So I decided that the compass simply must be FUBAR, started tracking with just the turn&bank, which didn't care about vacuum, and it appeared that straight & level was still working in the Twilight Zone after all. So, the patch came off the DG, subsequently confirmed to be just fine, and a dirty sock from my chart/underwear bag went over the compass for the duration.
All things being equal, I considered that haze to have been the most dangerous condition I had ever flown in, which takes in quite a few conditions, far preferring to be in solid IMC with others following the same rules and the shepherds on the ground watching over us, or even over dark, moonless, wastelands or water where at least I can see the anti-collision lights of my brethren. But that haze on a busy Saturday when others could have me bore-sighted, and all of us have had that happen to us in far better conditions at one time or another, was definitely a sobering experience. At the very least, flight following service is essential as well as all the instruments you are competent to use.
The epilogue to the story is that when I got back that afternoon and proudly announced my keen analysis and diagnosis to the manager, their A&P walked in and said, "Yeah, I haven't been able to set the compass card for a while and I've been meaning to get that one demagnetized. You wanna ferry it up there for us?"