Ruby embraces metaprogramming (reflection, introspection), multi-paradigm programming, and dynamism at an uncommon level. It's easy to shoot yourself in the foot with power and flexibility.
Troublesome? Ruby has the ability to be extremely readable or inscruptable. I've seen code that looks like it belongs in a Bash script.
Bad Practices? Some Rubyist value cleverness over wisdom. They write and share tricks that show off their cleverness, but this creates unreadable and fragile code.
As an aside:
@James and for loops...
When you do a for loop in ruby, it then calls ".each". Therefore, "for" is syntactic sugar for people more comfortable with C style loops. But as a Rubyist, you're going to use iterators like .map, .inject, .each_with_object, all the time. You'll never have to write a for loop with something like "i=0;i>6;i++" in ruby, and so you end up dropping the habit.
eloquent ruby doesn't endorse for loops.