I searched around and couldn't find a good consensus on the topic.
If I am trying to open a lock, and do not have thieves' tools, what detriment do I get?
From the thieves' tools. entry we have
Without these tools, you must use improvised tools, and you take a –2 circumstance penalty on Disable Device checks.
and the disable device page says for opening locks
The DC for opening a lock depends on its quality. If you do not have a set of thieves' tools, these DCs increase by 10.
So, do "improvised tools" from the thieves' tools page count as "thieves' tools" for the disable device check, making me only get a -2 penalty? Or do both count, giving me (in effect) a -12 penalty?
One gives a skill penalty for all disable rolls, the other raises lock DCs. The net effect is the same, but they are different modifiers. Like giving your opponent an attack penalty or giving you an ac boost vs missile attacks, both apply if the bad guy uses a bow.