The moment you buy something that exceeds the weight limit of the mech, it freezes.
Take for example a 50 ton Hunchie.
You remove, say 3 heatsinks and tries to add a medium laser.
This will cause it to hang.
Why?
This is because the mechlab simply could not calculate the 3 heatsinks being gone but adds the medium laser causing your hunchie to be 51 ton and hence, frozen mechlab.
Solution
What you should do is, remove 3 heatsinks, save the mech config.
Go back to mechlab again and add the medium laser.
Done.
Edited by Vach, 05 December 2012 - 04:56 AM.