There is probably nothing in the world of computing quite so scary as the serial port. Few who write serial drivers really know what is going on, and nothing can crash your machine in quite the same spectacular way as an application that accesses the serial port.
There is also nothing more difficult to test than a modem. Well over 10 years ago, a friend of mine and I tested the hottest new modems that were then available (they were 2400 bps).
We published a list on our BBS. We listed the modems and the software. We showed the speeds we got with each combination of modem and software. We sent files of specific size back and forth so that we could time exactly how long it took to send X-number of bytes. The rest was simple, or so we thought.
At 2400 bps one ought to be able to get just under 240 characters-per-second transfer. (The calculation is made by assuming seven or eight bits for the character and a parity bit. It comes close to 10 bits usually.) If you create a text of exactly 2,400 characters, it should take about 10 seconds to transfer. All our tests showed that it was taking about 12 or 15 seconds. People on the BBS - this was before the Internet boom - refused to believe us.
Typical comments were 'my manual says the modem works at exactly 2400 bps' and 'my high-speed light is on so it must be 2400 bps'. Finally we gave up and stopped publishing the lists.
The problem is not a lot better today. With modems that automatically step down if the line is bad, we often have no idea what the real speed is.
If a page takes a long time to download, it is often difficult to know if it is because of the modem of the Internet service provider (ISP), or some other server.