Many years ago I made the determination that if a saw cut crooked it was dull. Plain and simple, if you defined sharp as being able to cut properly (and having been sharpened properly) then a sharp saw cut straight. Years of hearing people beef "This bar's bent!" only to sharpen the saw and demonstrate that the saw now cut straight, "bent bar" or not.
Of course I ended up eating my words. I was using this old Stihl 090 with the gear reduction unit, pulling 1/2" pitch chain around a 6' smooth roller nose Canon bar. I was sawing some pretty big stuff, and the chain sure looked sharp. No matter what, it cut crooked, turning to the right. I finally gave in and filed the chain; it still cut crooked. I was puzzled. Checked the rakers ("stops"). That's when I noticed, the chain had been assembled from three or four short pieces. What clued me in was there was round profile cutters and square profile cutters on the same chain (Oregon 10). I got to looking, and everywhere the chain had been linked there were two cutters facing the same direction. As it turned out, all 4 splices had two cutters and they all faced the same way. Therefor you had 8 more cutters on one side than the other, so naturally, one side of the cut got cut faster than the other, therefor a crooked cut, even though the chain was filed correctly (hand filed witha 1/4" file, but I was pretty good at that sort of thing).
I bought a roll of 10BC Oregon chain, and made a loop of all the same size and type of teeth with only one master link. Now the chain cut straight.
That saw had a lot of memories. I used it on a fire once fallign trees. At the end of the day some helpful copter pilot dumped a bucket load of ocean water on me and my saw. I went on home and took a couple days off. When I came back, I didn't need the big 090 so I left it in the shop. About a week later I noticed the chain was pretty much rusted into one piece, attached to the bar. I'd have thought all the oil I maunally pumped onto it would have prevented that. I'd have been wrong. A little work and a bit of diesel got thigns loosened up.
One more 090 tale; the kill switch didn't work. To stop the saw, you could just choke it, or reach up and pull the plug wire off the top of the saw. There was an errant wire sticking out of the rubber boot. Many a nasty shock was received from this thing, until I got smart and started getting OTHER people whe wanted to help with this big saw, to pull the wire for me...