Music likes to work in sets. The most common is 4/4 where there's 4 measures in a set (or whatever the proper term for it is)
for 4/4 as an example. Count the big white bars.
There are 5 big white bars in this one. From the start of the first big white bar until just before the 5th white bar is that set. The 5th big white bar is the start of the second set.
While the majority of these issues are because they start the timing in the anacrusis (where lead-in usually plays) at the beginning of the song, it can also happen when it is cut short, or extended in some way past the normal 4/4.
Taking a basic 4/4 song for example. Out of all of mine, the only good one I found was an incomplete one lol.
http://puu.sh/yZZVQ.oszFirst off there's some noise before the main downbeat hits around 00:01:814 - and such. Some people may start timing off this or back at 00:01:237 - so they can map the lead-in noises here in the anacrusis. This means the set of 4 will be starting 1 measure early since in music it would start at 00:02:776 - where the strongest downbeat is and the song actually starts.
From here on, starting at 00:02:776 - start counting the white bars, 1,2,3,4 1,2,3,4. You'll notice some repetition on the second set. Since the next part can be confusing, jump back in at 00:27:391 - and start counting again. 00:27:391 (1) - and 00:33:545 (1) - are similar in this regard since it resets the set again. Usually these can be refered to as sections in the music.
So this is where it gets fun. If you start counting at 00:33:545 (1) - you'll get to 4, and 1 will be at 00:39:699 (1) - followed later by the reset at 00:41:237 (1) - Again this is basically just lead-in to the upcoming section, so a reset is needed here to get back on the proper 4/4 set. This happens again at 01:08:160 (1) - and 01:21:236 (1) -
Hopefully this somewhat helps. While this isn't unrankable, it does help out visual effects and mods like Nightcore play properly.