.001 difference from planned shouldn't make enough to cause cracking bottles. 1.020 is a bit higher than I like to bottle at, but depending on the recipe could be fine. I bottled my 999 Barleywine at 1.030 'cause the yeast were toast (at 17.5%!!!
) I've noticed that most of the extract kits I've done have finished fairly high, around 1.017-1.020 and no cracked bottles yet.
How hard did you lay on the capper? I have actually cracked the neck of a bottle while capping. I also got a few bad bottles that the neck snapped on when they were opened. The cap stayed attached and there was a very even break all the way around the neck. I really hated pouring those beers down the drain, but didn't want to chance that glass had gotten into them.