This is called "rapid-fire", and people usually notice it if they're monitoring or analysing an access log file.
First of all check if it is a problem by checking the load of your server, and monitoring your servers' error log, and concurrent connections if you can. If you have a medium or high performance server, it is quite likely to be able to cope a high load of even several requests per second, especially if the visits are quick.
However you may have problems if you have a low performance site, such as your own desktop PC or Mac you're working on, or you run low performance server software, or if you have many long retrievals (such as CGI scripts or large documents). These problems manifest themselves in refused connections, a high load, performance slowdowns, or in extreme cases a system crash.
If this happens, there are a few things you should do. Most importantly, start logging information: when did you notice, what happened, what do your logs say, what are you doing in response etc; this helps investigating the problem later. Secondly, try and find out where the robot came from, what IP addresses or DNS domains, and see if they are mentioned in the list of active robots. If you can identify a site this way, you can email the person responsible, and ask them what's up. If this doesn't help, try their own site for telephone numbers, or mail postmaster at their domain.
If the robot is not on the list, mail me with all the information you have collected, including actions on your part. If I can't help, at least I can make a note of it for others.