I get this every time [EDIT: I've been unable to reproduce this since the hotfix this morning - 11/8/2012 ~1030hrs Pacific Standard Time, GMT-0800] on my Centurion and Hunchback when purchasing new items and/or changing the loadout in some way.
To reproduce it, I have to change the mech configuration in a way that results in a purchase. It even happens if the only purchase is ammo - it doesn't matter /what/ is purchased.
After I confirm the purchase, I get a screen with the mech but no configuration, and it doesn't go back to the overview (in other words, I'm still zoomed into the mech, as if there's an invisible configuration window open or somesuch). If I return to the home screen and come back to the mech bay, it disappears my owned mechs.
Like others have said, if I quit and restart the game, my mechs are back, and all my purchases are available. However, my configuration changes that resulted in the purchase aren't saved, so I have to redo the configuration, armor loadout, etc. Kind of tedious.
I'll try to get a video of it and post it after the servers are back up.
EDIT: Unable to reproduce since the latest hotfix. Odd, as I thought they only fixed LRMs. Maybe I need to play a match before it will occur.
EDIT2: Still unable to reproduce, even after playing a match.
Edited by Wireball, 08 November 2012 - 10:58 AM.