Error: unexpected symbol/input/string constant/numeric constant/SPECIAL in my code

These errors mean that the R code you are trying to run or source is not syntactically correct. That is, you have a typo.

To fix the problem, read the error message carefully. The code provided in the error message shows where R thinks that the problem is. Find that line in your original code, and look for the typo.

Prophylactic measures to prevent you getting the error again

The best way to avoid syntactic errors is to write stylish code. That way, when you mistype things, the problem will be easier to spot. There are many R style guides linked from the SO R tag info page. You can also use the formatR package to automatically format your code into something more readable. In RStudio, the keyboard shortcut CTRL + SHIFT + A will reformat your code.

Consider using an IDE or text editor that highlights matching parentheses and braces, and shows strings and numbers in different colours.

Common syntactic mistakes that generate these errors

Mismatched parentheses, braces or brackets

If you have nested parentheses, braces or brackets it is very easy to close them one too many or too few times.

{}}
## Error: unexpected '}' in "{}}"
{{}} # OK

Missing * when doing multiplication

This is a common mistake by mathematicians.

5x
Error: unexpected symbol in "5x"
5*x # OK

Not wrapping if, for, or return values in parentheses

This is a common mistake by MATLAB users. In R, if, for, return, etc., are functions, so you need to wrap their contents in parentheses.

if x > 0 {}
## Error: unexpected symbol in "if x"
if(x > 0) {} # OK

Not using multiple lines for code

Trying to write multiple expressions on a single line, without separating them by semicolons causes R to fail, as well as making your code harder to read.

x + 2 y * 3
## Error: unexpected symbol in "x + 2 y"
x + 2; y * 3 # OK

else starting on a new line

In an if-else statement, the keyword else must appear on the same line as the end of the if block.

if(TRUE) 1
else 2
## Error: unexpected 'else' in "else"    
if(TRUE) 1 else 2 # OK
if(TRUE) 
{
  1
} else            # also OK
{
  2
}

= instead of ==

= is used for assignment and giving values to function arguments. == tests two values for equality.

if(x = 0) {}
## Error: unexpected '=' in "if(x ="    
if(x == 0) {} # OK

Missing commas between arguments

When calling a function, each argument must be separated by a comma.

c(1 2)
## Error: unexpected numeric constant in "c(1 2"
c(1, 2) # OK

Not quoting file paths

File paths are just strings. They need to be wrapped in double or single quotes.

path.expand(~)
## Error: unexpected ')' in "path.expand(~)"
path.expand("~") # OK

Quotes inside strings

This is a common problem when trying to pass quoted values to the shell via system, or creating quoted xPath or sql queries.

Double quotes inside a double quoted string need to be escaped. Likewise, single quotes inside a single quoted string need to be escaped. Alternatively, you can use single quotes inside a double quoted string without escaping, and vice versa.

"x"y"
## Error: unexpected symbol in ""x"y"   
"x\"y" # OK
'x"y'  # OK  

Using curly quotes

So-called "smart" quotes are not so smart for R programming.

path.expand(“~”)
## Error: unexpected input in "path.expand(“"    
path.expand("~") # OK

Using non-standard variable names without backquotes

?make.names describes what constitutes a valid variable name. If you create a non-valid variable name (using assign, perhaps), then you need to access it with backquotes,

assign("x y", 0)
x y
## Error: unexpected symbol in "x y"
`x y` # OK

This also applies to column names in data frames created with check.names = FALSE.

dfr <- data.frame("x y" = 1:5, check.names = FALSE)
dfr$x y
## Error: unexpected symbol in "dfr$x y"
dfr[,"x y"] # OK
dfr$`x y`   # also OK

It also applies when passing operators and other special values to functions. For example, looking up help on %in%.

?%in%
## Error: unexpected SPECIAL in "?%in%"
?`%in%` # OK

Sourcing non-R code

The source function runs R code from a file. It will break if you try to use it to read in your data. Probably you want read.table.

source(textConnection("x y"))
## Error in source(textConnection("x y")) : 
##   textConnection("x y"):1:3: unexpected symbol
## 1: x y
##       ^

Corrupted RStudio desktop file

RStudio users have reported erroneous source errors due to a corrupted .rstudio-desktop file. These reports only occurred around March 2014, so it is possibly an issue with a specific version of the IDE. RStudio can be reset using the instructions on the support page.

Using expression without paste in mathematical plot annotations

When trying to create mathematical labels or titles in plots, the expression created must be a syntactically valid mathematical expression as described on the ?plotmath page. Otherwise the contents should be contained inside a call to paste.

plot(rnorm(10), ylab = expression(alpha ^ *)))
## Error: unexpected '*' in "plot(rnorm(10), ylab = expression(alpha ^ *"
plot(rnorm(10), ylab = expression(paste(alpha ^ phantom(0), "*"))) # OK

For me the error was:

Error: unexpected input in "�"

and the fix was opening the script in a hex editor and removing the first 3 characters from the file. The file was starting with an UTF-8 BOM and it seems that Rscript can't read that.

EDIT: OP requested an example. Here it goes.

➜  ~ cat a.R
cat('hello world\n')
➜  ~ xxd a.R
00000000: efbb bf63 6174 2827 6865 6c6c 6f20 776f  ...cat('hello wo
00000010: 726c 645c 6e27 290a                      rld\n').
➜  ~ R -f a.R        

R version 3.4.4 (2018-03-15) -- "Someone to Lean On"
Copyright (C) 2018 The R Foundation for Statistical Computing
Platform: x86_64-pc-linux-gnu (64-bit)

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under certain conditions.
Type 'license()' or 'licence()' for distribution details.

  Natural language support but running in an English locale

R is a collaborative project with many contributors.
Type 'contributors()' for more information and
'citation()' on how to cite R or R packages in publications.

Type 'demo()' for some demos, 'help()' for on-line help, or
'help.start()' for an HTML browser interface to help.
Type 'q()' to quit R.

> cat('hello world\n')
Error: unexpected input in "�"
Execution halted

If you are copy-pasting code into R, it sometimes won't accept some special characters such as "~" and will appear instead as a "�". So if a certain character is giving an error, make sure to use your keyboard to enter the character, or find another website to copy-paste from if that doesn't work.