We have found out that small percentage of RB800 (less than 10%) does have heating issues when miniPCI slots are populated. Units, that are reported to have this problem by users, should be returned for replacement to MikroTik.
A later advice:
Document on link below describes two possible solutions for the RB800 devices that have issues during excess ambient heat (device hangs when ambient temperature reaches around 65C or 150F).
Please note that only a small number of RB800 devices are affected (around 10%), and new devices with fixes applied are already shipping from MikroTik warehouses as of 01-JUL-2010. Future production batches already include design changes so that no fixes are necessary for full functionality.
Customers are asked to return affected units to RMA at the local distributors for a replacement unit, however, if they are experienced in soldering, and need a quicker solution, they can follow fix that's described at: