Reserved Names for Ruby on Rails.. Feel free to comment others you might think of and I'll add it in!

Don't Use These Names!

ADDITIONAL_LOAD_PATHS
ARGF
ARGV
ActionController
ActionView
ActiveRecord
ArgumentError
Array
Base
BasicSocket
Benchmark
Bignum
Binding
CGI
CGIMethods
CROSS_COMPILING
Class
ClassInheritableAttributes
Comparable
ConditionVariable
Config
Continuation
DRb
DRbIdConv
DRbObject
DRbUndumped
Data
Date
DateTime
Delegater
Delegator
Digest
Dir
ENV
EOFError
ERB
Enumerable
Errno
Exception
FALSE
FalseClass
Fcntl
File
FileList
FileTask
FileTest
FileUtils
Fixnum
Float
FloatDomainError
GC
Gem
GetoptLong
Hash
help
IO
IOError
IPSocket
IPsocket
IndexError
Inflector
Integer
Interrupt
Kernel
LN_SUPPORTED
LoadError
LocalJumpError
Logger
Marshal
MatchData
MatchingData
Math
Method
Module
Mutex
Mysql
MysqlError
MysqlField
MysqlRes
NIL
NameError
NilClass
NoMemoryError
NoMethodError
NoWrite
NotImplementedError
Number
Numeric
OPT_TABLE
Object
ObjectSpace
Observable
Observer
PGError
PGconn
PGlarge
PGresult
PLATFORM
PStore
ParseDate
Precision
Proc
Process
Queue
RAKEVERSION
RELEASE_DATE
RUBY
RUBY_PLATFORM
RUBY_RELEASE_DATE
RUBY_VERSION
Rake
RakeApp
RakeFileUtils
Range
RangeError
Rational
Regexp
RegexpError
Registration
Request
Response
RuntimeError
STDERR
STDIN
STDOUT
ScanError
Scope
ScriptError
SecurityError
Session
Signal
SignalException
SimpleDelegater
SimpleDelegator
Singleton
SizedQueue
Socket
SocketError
StandardError
String
StringScanner
Struct
Symbol
SyntaxError
SystemCallError
SystemExit
SystemStackError
TCPServer
TCPSocket
TCPserver
TCPsocket
TOPLEVEL_BINDING
TRUE
Task
Template
Text
Thread
ThreadError
ThreadGroup
Time
Transaction
TrueClass
type
TypeError
UDPSocket
UDPsocket
UNIXServer
UNIXSocket
UNIXserver
UNIXsocket
UnboundMethod
Url
VERSION
Verbose
YAML
ZeroDivisionError