Developing on your local machine instead of a webserver saves a developer a lot of time and allows you to test functionality without the need for a test site. There are a couple of tools to allow someone to set up a server on their local machine; one of the most common tools is MAMP.
In using MAMP for a couple years there are a few gotcha's that I've run into that are actually quite easy to fix if you know what to look for.
Apache Server Not Starting
If Apache isn't starting is could be that Apache may already be running. You can turn off Apache manually in the terminal.
The other problem that I've had is when Apache is set through MAMP to open a particular folder and that folder no longer exists. If you simple go to MAMP and change that setting through the preferences button you should be able to fix it.
MySQL Not Starting
If MySQL isn't starting it is pretty likely that it could already be running. You can very easily manually turn it off in the terminal. I've had this happen to me a few times and I believe it has something to do with not shutting MAMP off correctly.
MySQL Error #2002 – The server is not responding
You sometimes get this problem on the phpMyAdmin page when you've had another MySQL problem. This is actually just a caching problem. Just add a query string to your url or clear your cache and it should fix itself.
Conclusion
These are the only problems I've had with MAMP and they are quite easily fixed. Hopefully these little tips will save you some time.