This happened a couple of decades back. I was asked by a computer dealer to appear in court as an expert witness. They wanted me to swear that a 10 mbit ethernet network, is 2.5 times as fast as a 4 mbit token ring network. They had replaced, for a customer, their old Token Ring network with a new ethernet network, and the customer was complaining that it ran very very slowly now. And this was going to be a court case, because customer wanted dealer to rip out the ethernet cabling and hardware, and reinstall their old token ring, and that would be very expensive for the dealer.
It sounds like it would be faster. But networks aren't quite that simple. And why would the customer be wrong about this? So I said "I need to visit and have a look at this." "The dealer said, "Why? It's very simple, 10 mbit compared to 4 mbit." So I explained, when I'm up on the witness stand giving my expert opinion, and the cross-examination asks me "So, Dr Solomon, did you actually go and look at this network?" then I don't want to have to ansewr, "No, I'm just guessing."
So we made an appointment, and I went to visit the customer. The customer showed me; he started up Dataease (a database that I knew and loved) and nothing happened, for a very very long time, until eventually, the opening screen came up. "Odd, I though, "that should have been instant, they haven't actually done anything yet." So we closed it down, and started it up, and this time I went to watch the server. And the disk light was flashing, and flashing, and flashing - lots of disk access, and eventually the database started up.
"Hmmm, I thought, "I wonder what's causing that?" So I started it up again, but in single user mode, and this time, it came alive instantly. The customer was watching, and said "Yes, that's more like it! Well done, you've fixed it." No, I hadn't. You can't use a multi-user database in single user mode, there will be collisions. But it was clear what was wrong; the database was implementing locking by opening little files to indicate "File in use", and closing them, and opening them, and so on.
So, how to fix this? I phoned Dataease tech support; I had a good name to talk to, which got me past the front line support "Have you switched the monitor on?". And he said, "Yes, that was version 4, but version 5 does proper in-memory locking using the operating system locks." "So how can I get an update?" I asked. And the customer interrupted, "Do you mean this?" he asked, waggling a copy of Dataease 5 that they had been sent but hadn't bothered to install.
So I upgraded their software, ran it multiuser, it was swift as an arrow, everyone was happy (the customer, the dealer and me - but not the lawyers who had been deprived of their fees).