Strings containing MD5 hashes or vanilla alphanumeric values seem to work fine, but using a string that contains a URL sometimes appears to cause indexing to fail.
Jul 31 05:36:37 Timmy-OCrashington Art Authority[519] <Warning>: [com.apple.corespotlight.log.index] Error caught while talking to the index agent, index:<CSSearchableIndex:0x145b01d70; name="CSSearchableIndexShared">, error:Error Domain=NSCocoaErrorDomain Code=4097 "connection to service named com.apple.spotlight.IndexAgent" UserInfo={NSDebugDescription=connection to service named com.apple.spotlight.IndexAgent}
Jul 31 05:36:37 Timmy-OCrashington Art Authority[519] <Warning>: [com.apple.corespotlight.log.index] Finished "index items"(16) with error:Error Domain=CSIndexErrorDomain Code=-1003 "(null)"
Jul 31 05:36:37 Timmy-OCrashington Art Authority[519] <Warning>: [com.apple.corespotlight.log.index] Error caught while talking to the index agent, index:<CSSearchableIndex:0x145b01d70; name="CSSearchableIndexShared">, error:Error Domain=NSCocoaErrorDomain Code=4097 "connection to service named com.apple.spotlight.IndexAgent" UserInfo={NSDebugDescription=connection to service named com.apple.spotlight.IndexAgent}
Jul 31 05:36:37 Timmy-OCrashington Art Authority[519] <Warning>: [com.apple.corespotlight.log.index] Finished "index items"(14) with error:Error Domain=CSIndexErrorDomain Code=-1003 "(null)"