A common sort of patch received by open source project maintainers is the "paper towel roll" patch. It's a patch that was coded while its author looked through a paper towel roll at some very specific bit of code in your larger system. The patch is wrong, but its author does not have enough context to know why: it patches some subsystem in a way doesn't make any sense when the entirety of the system is considered. It works, but applying it as-is would be disastrous on some level (documentation requirements / conceptual integrity / code cleanliness, additional unwanted software dependencies, etc).
Paper-towel-roll patches are tricky to deal with as an open source maintainer. Do you throw it away? Do you add the feature implied by the patch in "the right way"? How do you deal with the original submitter? How pissed off is he going to be if you reject it out-of-hand without trying to help him implement it in the right way? Do you even want the feature? Even if you're +0 or +1 on the feature, do you have enough time to deal with doing it properly?
OTOH, a pissed off contributor isn't the end of the world, so it isn't as big a problem as some people think, even in the worst case.