... and coded the functionality "manually"?
As a rather metaphorical example, you'd hardly need a library for solving quadratic equations, although such libraries do exist and someone takes them seriously.
As a more questionable case, depending on the circumstances I may ditch jQuery (for example when I don't need to support some stone age browsers): it does simplify some things but it adds another layer of complexity and unreliability to DOM. And overusing jQuery leads to absurd problems, such as one seen recently on SO: how do I assign an empty href to an
Another absurd case and yet non-obvious for many is using some templating engine/language built on top of another templating system: PHP. Third level of templating anyone?
And another one: sometimes just spitting out XML with (figurally)
printf is far easier than doing it with some monstrous XML engine.
Any other cases from your experience?