Sometimes they are. It depends on how the capture was generated. If you look in the options of Wireshark there is one to detect bad checksums, so clearly there is a way to capture them. Here is one such way: https://stackoverflow.com/questions/22101650/how-can-i-recei...
This can be used to detect partially-bad network cables, because there is no reason you should ever receive a bad FCS.
This can be used to detect partially-bad network cables, because there is no reason you should ever receive a bad FCS.