saving screws up my tracks

so i was making a track, using version 6-17-12, and fp9 (I think, long story), and my track was working well. I saved it and quit, and when I loaded it back up to work on it, it no longer worked at the part I was working on. Is it cause the rest of the track may have been made in a different version? I think I made it in 6.3. Any help would be much appreciated.

FP9 has different phsyics than FP10/11
Also 6.7 works differently than other versions sometimes

^ That depends which build of 6.7, I believe. Mhenr used to use a decompiled version of the code that had very minute differences, but ended up remaking it with the actual source code he managed to nab from InXile in the middle of the night by smashing cameras and killing a few security guards.

^ but it is okay though, since he's australian