Magento is a great webshop with endless possibilites. However, its dynamic structure, and the way modules are written can give some headache.
The problem can be that two or more modules inherit the same core class. Also called Magento module conflict. When that happens, one of the modules wins, and the rest looses. You do not really which one wins, and the result can be strange behaviour and features not working as expected.
How to detect a Magento module conflict?
Its quite easy to detect, if you have any conflicts in Magento, without waiting for some customer reporting that something is not working. You can install this module http://www.magentocommerce.com/magento-connect/modules-conflict-detector.html
It will tell you if you have any conflicts, if everything is green and fine you do not have any conflicts.
How to fix a Magento module conflict?
Several solutions exist. But you need to get your fingers into the code, unless you uninstall one of the conflicting modules and use something else.
The solution can be to let the one conflicting class inherit the other conflicting class. You need to use the Magento dependency concept, where you can make sure that one module loads before another, to make sure which one should inherit the other.
Another less attractive approach is to look at the two classes that conflicts, and write them together into one class. Its not very nice, and if one of the modules gets updated the result can be the conflict getting back.
You can read more about Magento module conflict resolving here http://www.webshopapps.com/blog/2010/11/resolving-magento-extension-conflicts/
Conclusion is, that it can be tricky to fix a Magento module conflict, you might need to consult an expert, or find other alternative modules. However, its easy to detect if you have a potential problem, you can do that with the module described in the first paragraph of this blog.