First of all im sorry for my bad english,im trying my best ^^ So the bug is kinda hard to explain.. when i run up the slabs, i try to jump of directly after i reach the top where the slabs end but instead of jumping immediately, i start jumping 3-4 blocks later...
it doesnt happen everytime, its acutally pretty rare to me like from 50 jumps it happens once or twice, but its still pretty annoying when youre getting chased

i tried this out in survival and in creative, in both versions, 1.7.2 and 1.6.4 it happened to me