Bad Thing: a behavior that has no (some might qualify: "easily seen") positive benefits.
For example, it can be a bad thing to mix Tcl list- and string-oriented operations against variables. Unless the developer is very careful, the program will produce unexpected and unwanted results, such as slow performance, unintended list expansion, and malformed list errors.
Another is to manually maintain multiple copies of data. One possibility is that they get out of sync. The other possibility is that they don't get out of sync, but that's only if time is wasted keeping them obsessively-compulsively sync'ed. Both possibilities are Bad; please don't do that. :^) See damage by duplication for a discussion of this phenomenon as it relates to this very wiki.
The common response to a bad thing is: don't do that.