If a function is simple enough that "buggy" can be defined in isolation, AI or a junior developer can probably write it without much supervision. Most bugs that escape the first-level developer are violations of contextual expectations: cases where the code would work as expected in a different application, with a different use case, or something like that. So the characteristics of "a buggy function" depend on the code, processes and users around it, and that is where junior developers often fall short.
Defensive programming or robustness and "good taste" reduce the frequency of those bugs, but knowing how to do those well usually comes from experience with mistakes in the general domain: for example, handling very long lines in line-oriented (text) input.
(Or, from my life last week, populating a "source identifier" field in a protocol. The code was targeted for use in two executables: one with a fixed source ID across all deployments, and one where four instances within the same process would each have their own source ID. For some reason, the software team decided to read the source ID from a config file -- and then never set up the config file, so it sent a zero ID. The protocol reserves zero values because somebody knew people will fuck up exactly in that kind of way. In a different environment, that module of code would have been fine, but it was buggy in the actual environment.)