OS is Windows 7 Professional x64 SP1.
Anti-virus was shut down at the time (I stopped the service, shut down Bandicam and Steam, then relaunched Bandicam and Steam to ensure no hooks remained open to their processes).
There was no other software running that displays information over the game window or hooks DirectX or the game processes in any way that I am aware of.
With GTA V, the Steam overlay simply doesn't work (can't be opened and never displays its little notification in the lower-right of the screen), however the game is still playable and Bandicam still works OK.
Star Trek Online will crash as soon as the Steam overlay tries to display its notification that you can press Shift+Tab to "access the Steam community" in the lower-right of the game window. This only happens when Bandicam is running. I did allow their crash reporter to submit a report about the crash.
To debug this issue, I added the following to the "Blocked process list", on the assumption that this list prevents Bandicam from opening hooks to those processes:
I also disabled the option that says "Block DirectX video overlay" in the Hooking options.
None of this seemed to have any effect.
I have verified that the Steam overlay works OK with Red Faction: Guerrilla while Bandicam is running, so it doesn't have issues with every game. I have yet to try most of my games to see which ones I do and do not have problems with.