It's common for CPU coolers (or the fan at least) to overhang the first RAM slot, usually you can still fit lower-profile DIMMs in there but you'll have to either remove the fan or in some cases the whole cooler. If you're using RAM with daft heatsinks on then you're more likely to have issues.

You can always turn the fan on the cooler around to blow in the opposite direction, however it's usually best left in it's default configuration.