We are talking files on the order of gigabytes per second. They can get real big real quick and fill up your memory like no tomorrow.
If you're wondering the math, each frame is about 40 mb in a tga sequence. If you're recording at 1080p 60 fps you're looking at 2.4 gigabytes per second. Using lagarith which is a lossless codec, it's a bit better but still quite large.