I think interplanetary missions usually start by reaching low earth orbit, and only then burning at the right time to start the interplanetary transfer. So getting stuck in LEO is not that unlikely.

For example, the Russian Fobos-Grunt[1] mars mission got stranded in LEO. Though indeed not in a stable orbit; it did re-enter and burn up two months later.

[1] https://en.wikipedia.org/wiki/Fobos-Grunt