If it's DF2, there's too many items in the map, start by removing items till the sys dump stops.
If it's DFLW, do not use any items that starts with at "d" such as dsatch or dsatchel - these are decorations and will cause a sysdump. Likewise placing clays or satchels in a map and attempting to use them as land mines - once they explode they will boot everyone from your server.
If it's DF:TFD then you're using one of the buildings that is not available and this also will cause a sysdump. I believe Stevie has a list of buildings for TFD that are no good.
As Wildman asked, it's advisable to post what game you're refering to.
Oh also.. just remembered, if it's for TFD and you're using a NIGHT map, DO NOT!!! Select or check the box for NIGHT VISION! This will cause a sys dump every time. To get around this make the night map as usual and then leave the night vision turned off. Once the map is up and running, those who enter can turn on their NV and all will be ok.
