Opera Mini uses the phone's RAM to temporarily store pages. It has nothing to do with the phone's internal or memory cards memory. Once the phone's RAM is filled, the phone starts giving memory low errors. This is more common in phones with low RAM. If you are using Opera Mini, then set the image quality to Medium or Low. This way the memory won't fill up quickly. If it does, then you'll have to exit and restart Mini to empty the RAM again. The RAM will also fill up quicker if you have the Music player or FM radio app running in the background.