Which characters make a URL invalid?

The '[' and ']' in this example are "unwise" characters but still legal. If the '/' in the []'s is meant to be part of file name then it is invalid since '/' is reserved and should be properly encoded:

http://example.com/file[/].html

To add some clarification and directly address the question above, there are several classes of characters that cause problems for URLs and URIs.

There are some characters that are disallowed and should never appear in a URL/URI, reserved characters (described below), and other characters that may cause problems in some cases, but are marked as "unwise" or "unsafe". Explanations for why the characters are restricted are clearly spelled out in RFC-1738 (URLs) and RFC-2396 (URIs). Note the newer RFC-3986 (update to RFC-1738) defines the construction of what characters are allowed in a given context but the older spec offers a simpler and more general description of which characters are not allowed with the following rules.

Excluded US-ASCII Characters disallowed within the URI syntax:

   control     = <US-ASCII coded characters 00-1F and 7F hexadecimal>
   space       = <US-ASCII coded character 20 hexadecimal>
   delims      = "<" | ">" | "#" | "%" | <">

The character "#" is excluded because it is used to delimit a URI from a fragment identifier. The percent character "%" is excluded because it is used for the encoding of escaped characters. In other words, the "#" and "%" are reserved characters that must be used in a specific context.

List of unwise characters are allowed but may cause problems:

   un