[weaksauce]

Do you have an example of such code that does something off label or tricky to intuit?

[zrail]

Active record hooks. Observers especially are a spooky action at a distance factory.

[thih9]

Altering expected framework functionality. E.g. a custom getter method for an ActiveRecord attribute.

Technically this isn't a Rails-only thing; but since Rails relies so much on convention, there are many opportunities to break it (and possibly bring suffering to other devs).

[emodendroket]

Besides the other examples already given, monkey patching is another offender, as are proxy tricks with methods like send or respond_to (technically not Rails-specific but I associate this style with Rails).