Limitations of Language

If you take a closer look, you'll notice that Ruby's grammar has quite a few edge-case where its syntax is inconsistent or ambigious:

Binary Minus vs Minus taken as Unary Method Argument

>> [1,3,4,5].size - 1
# => 3

>> [1,3,4,5].size -1
# wrong number of arguments(1 for 0)

No Simple Rule, if a Symbol can be Displayed Without the Explicit :"" Syntax

>> {:< => 0}
# => {:<=>0}

>> {:<=>0}
# syntax error, unexpected tINTEGER,
# expecting tASSOC

Different Rules for Modules and Classes

>> module Eurucamp end
# => nil

>> class Eurucamp end
# syntax error, unexpected $end

Update: Since Ruby 2.3 this is no longer invalid syntax:

>> class Eurucamp end
# => nil

Ambiguous if to Parse ~ Unary or as Part of Binary Match Operator

>> $_="Eurucamp X"
>> ~/X/ # => 9

>> a=~/X/ # undefined local variable or method `a'
>> a= ~/X/ # => 9

Global Variables can "Break" String and Regex Syntax

>> a = "Eurucamp #\n"
>> a.gsub /#$/, ''
# => ?

>> a = "Eurucamp #\n"
>> a.gsub /#$/, ''
# unterminated regexp meets end of file

>> a = "Eurucamp #\n"
>> a.gsub /#$//, ''
# => "Eurucamp #"

Unary or Binary Plus, Part 2

>> p = 21
>> p 21
# => 21

>> p = 21
>> p +21
# => 42

String Creation vs Format Method

>> puts%[1]
# undefined method `%' for nil:NilClass

>> puts %[1]
# => 1

Hash vs Block

Ruby uses curly braces for both, blocks as well as hashes. Sometimes, this leads to confusing cases:

>> def identity(a) a end

>> identity(1)
#=> 1
>> identity 1
#=> 1
>> identity({})
#=> {}
>> identity {}
#=> wrong number of arguments (0 for 1) (ArgumentError)

Regex vs Division

>> puts /4/i
# => (?i-mx:4)

>> puts, i = 42, 2
>> puts /4/i
# => 5

Further Reading

More Idiosyncratic Ruby