Index: src/OFExceptions.h
==================================================================
--- src/OFExceptions.h
+++ src/OFExceptions.h
@@ -884,5 +884,11 @@
 /**
  * An OFException indicating that unlocking a mutex failed.
  */
 @interface OFMutexUnlockFailedException: OFException {}
 @end
+
+/**
+ * An OFException indicating that the hash has already been calculated.
+ */
+@interface OFHashAlreadyCalculatedException: OFException {}
+@end

Index: src/OFExceptions.m
==================================================================
--- src/OFExceptions.m
+++ src/OFExceptions.m
@@ -1263,8 +1263,22 @@
 		return string;
 
 	string = [[OFString alloc] initWithFormat:
 	    @"A mutex could not be unlocked in class %s", [class_ className]];
 
+	return string;
+}
+@end
+
+@implementation OFHashAlreadyCalculatedException
+- (OFString*)string
+{
+	if (string != nil)
+		return string;
+
+	string = [[OFString alloc] initWithFormat:
+	    @"The hash has already been calculated in class %s and thus no new "
+	    @"data can be added", [class_ className]];
+
 	return string;
 }
 @end

Index: src/OFHashes.m
==================================================================
--- src/OFHashes.m
+++ src/OFHashes.m
@@ -143,12 +143,11 @@
 
 	if (size == 0)
 		return self;
 
 	if (calculated)
-		/* FIXME: Maybe a new exception would be better */
-		@throw [OFInvalidArgumentException newWithClass: isa];
+		@throw [OFHashAlreadyCalculatedException newWithClass: isa];
 
 	/* Update bitcount */
 	t = bits[0];
 	if ((bits[0] = t + ((uint32_t)size << 3)) < t)
 		/* Carry from low to high */
@@ -382,12 +381,11 @@
 {
 	if (size == 0)
 		return self;
 
 	if (calculated)
-		/* FIXME: Maybe a new exception would be better */
-		@throw [OFInvalidArgumentException newWithClass: isa];
+		@throw [OFHashAlreadyCalculatedException newWithClass: isa];
 
 	sha1_update(state, &count, buffer, buf, size);
 
 	return self;
 }

Index: src/OFThread.m
==================================================================
--- src/OFThread.m
+++ src/OFThread.m
@@ -45,11 +45,10 @@
   forTLSKey: (OFTLSKey*)key
 {
 	id old = of_tlskey_get(key->key);
 
 	if (!of_tlskey_set(key->key, [obj retain]))
-		/* FIXME: Maybe another exception would be better */
 		@throw [OFInvalidArgumentException newWithClass: self
 						       selector: _cmd];
 
 	[old release];
 

Index: tests/OFHashes.m
==================================================================
--- tests/OFHashes.m
+++ tests/OFHashes.m
@@ -54,8 +54,15 @@
 	[f close];
 
 	TEST(@"-[digest]",
 	    !memcmp([md5 digest], testfile_md5, OF_MD5_DIGEST_SIZE) &&
 	    !memcmp([sha1 digest], testfile_sha1, OF_MD5_DIGEST_SIZE))
+
+	EXPECT_EXCEPTION(@"Detect invalid call of -[updateWithBuffer] #1",
+	    OFHashAlreadyCalculatedException, [md5 updateWithBuffer: ""
+							     ofSize: 1])
+	EXPECT_EXCEPTION(@"Detect invalid call of -[updateWithBuffer] #2",
+	    OFHashAlreadyCalculatedException, [sha1 updateWithBuffer: ""
+							      ofSize: 1])
 
 	[pool release];
 }