User summary:
Details from: Wikipedia, "Commodore Datasette", available under the CC-BY-SA License.

The Commodore Datassette (sometimes spelled Datasette) was the standard loading peripheral for the VIC-20 and the C64/128, and was always delivered together with the home computer in question.

The tape head was sensitive and varied between models, so the ?Load Error error when checksums didn't match up was a very common problem for all these machines. The tape head could be adjusted, but on earlier models was a hazzle with opening up the chassi and experimenting. Later models came with a pre-drilled hole to apply a screwdriver to, and third-party manufacturers applied a small screw and some LED-lights to imply signal quality.