Rust.
I use a 100m tape for more general marking duties in addition to measuring for track and field (say soccer pitches, just because not everyone has a 100m tape);
any tape that gets used through wet/damp grass needs to let any moisture out, like the plastic open reel ones; most metal cases are pretty cheaply made so lots of edges are uncoated/painted and the surface rust gets started fast which is messier to use later on.