In a twisted pair ethernet cable, it is important to not split the pairs. One pair each to pin 1 and 2, to pin 3 and 6, to pin 4 and 5, and the last pair to 7 and 8.
If you split one of these pairs, for example you just do 1/2, 3/4, 5/6, 7/8, a cheap continuity tester will not find the error if you do it like that on both ends. But network performance will be severly limited, if you even get a connection at all.
Also, as far as i know, the twist ratio for each pair is different, which can cause issues if you swap pairs.
I have personal experience with dicky connections, that could not be located with a cheap tester. The cheap crap claimed the cable is ok, but i could not even get a 100 Mbit/s link. Getting the big guns out, i.e. the cable certifier, immediately found the issue.