This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
CD recorders (and modern CD-ROM drives) have a chunk of RAM that holds
blocks read from the disc. Some drives provide a way to clear this out,
All drives need to have their block cache cleared out after writing
completes and before disc verification begins. If this weren't done,
the files being verified could be read out of the block cache instead
of from the disc itself, defeating the purpose of the verification pass.
Also, some CD recorders need to have their recording buffers explicitly
cleared between the "test" and "write" passes.
The most reliable, 100%-guaranteed-to-work approach is to eject the
disc and re-insert it. Watching your CD tray open and close can be
startling at first, but in general it's harmless.
Back in the early days of CD recording, the situation was a bit more
awkward. Caddy drives were the norm, so an ejected disc had to be
manually re-inserted. Some poorly-written CD recording software would
automatically start the "write" pass a few seconds after the "test" pass,
without waiting for the disc to be re-inserted, so you either had to be
paying close attention or set the "wait until told to continue" option.