I know (because of this question) that mixing CAT5e and CAT6 is not supposed to matter, but when installing a wired network I did run into the following problem - persistenty:
When adding a CAT6 extension cable to a CAT5e cable running from my modem to a switch elsewhere in the house, the wired speed was only 10 Mb;
When omitting that extension cable there and instead adding the very same cable to another CAT5e cable running from the switch to yet another room in the house, the speed was 100 Mb as it's supposed to be (for my modem).
Riddle me this! The extension cable is not at fault, obviously. The only theory I have is that the modem detected something funny on its outgoing cable and decided to drop the speed - but is that even possible?
(The speeds of 10 resp. 100 Mb were as reported by the modem, but confirmed by a manual speed test.)