There is a lot of misinformation in some answers. I will give the facts.
No MagSafe adapter, when plugged into a mechanically matching receptacle on a MacBook/MacBook Pro, will cause an unsafe condition. This is a given for the systems to receive safety certifications. So no, a 60W adapter won't overheat when connected to a machine that needs an 85W adapter. The MacBook it's plugged into won't operate in a "brownout". It will operate safely, but the CPU performance will be diminshed. The below explains why.
A MacBook's power management works very simply: it maintains a balance of power between the power consumed by the loads and the power available from the sources. There are two sources of power:
- power adapter,
- battery as a power source.
There are two loads:
- battery charger,
- the machine (logic, drives, memory, screen, speakers, USB devices, etc.)
Both loads are adjustable and the power management's function is to adjust them as needed.
The battery as a power source is exclusive of the battery charger: a battery may operate as a power source, or the battery charger may operate, but never both at once.
The power management must maintain the following inequality balanced, in terms of power: (power adapter + battery as a power source) >= (battery charger + the machine). The loads are prioritized: the machine has priority over the battery charger. The power management system also knows the electronic nameplate of the power supply and thus its rated power.
Thus, given an available input power, the machine load is satisfied first, and any leftover power is provided to the battery charger. If there isn't enough power left for the charger, the battery is by definition discharging unless it has no charge left. This is important. Conversely, a fully charged battery will demand a zero charger load, and that's fine.
If there isn't enough power for the machine, the load shedding kicks in and throttles the CPU (and perhaps GPU - I don't recall offhand). The CPU load shedding will, by design, always manage to balance the power. The 60W supply, even if connected to a 17 inch MBP, will satisfy all internal and external loads (USB, FW, drives, screen), except for the CPU and GPU. So the latter will be throttled to maintain the power balance. That's why the performance will be poor with an inadequate power supply.
Since the machine load takes priority and doesn't shed until there's insufficient power available, the battery will be always discharging whenever the supply can't provide sufficient power to cover the machine's needs. This means that with a 60W charger, the battery will charge only during light CPU load. If you have both cores going full-throttle, the battery will be always discharging until it reaches a zero charge state.
The rate at which the battery charges will also depend on the machine load. The battery charger can consume up to ~30W or so. With an 85W adapter, that leaves about 55W for the machine, and it's insufficient to power a full machine load. Since the machine load takes precedence, the power available to the charger will vary depending on the entirety of the machine load: CPU/GPU, drives, USB/FireWire, screen, etc. With a very high machine load, the charger is left with very little power to use, even with an 85W supply, and will take very long to charge the battery. The longest I've seen was 20+ hours with everything going full blast (full CPU+GPU load, all USB and FireWire ports delivering full rated power, all drives spinning, screen at full brightness, speakers blaring).
Finally, the supply's electronic nameplate is stored in the chip residing in the MagSafe jack. If the MagSafe jack is damaged or doesn't have the nameplate chip, the power manager does two things:
Assumes a 60W power supply.
Disables the battery charger.
