Gad, someone 'discovers' this every month. There's at least one big thread on it.
Another possibility, which is presented in both the original film and in the Frank Miller-written comic series "RoboCop vs. The Terminator", is that it's possible that the T-1000 was sent through time with a layer of cloned flesh on it.
Think about it, in the first film, it is established that the T-800 Terminator was able to go through because it had living tissue covering it. In "RoboCop vs. The Terminator", the final issue of the four part series, the Terminator-ized RoboCop goes back in time with a few missiles by covering himself and them with a huge chunk of cloned flesh.
It could be possible that there was a thin layer of cloned flesh on the T-1000 that didn't burn off until the chase scene from the Galleria (or possibly when the T-1000 exited out of the transport through time).
Tests.Here's another problem. If time travel in the "Terminator" universe is one-way, how did they know that nothing metal could go through?
Tests.
BTTF is a whole other time travel scenario... at least to the first Terminator movie's time travel, which is similar to the Twelve Monkey's time travel scenario: nothing can be changed, as the past has already happened. Sure, the terminator tried, but in effect, it just fulfilled it's true purpose in history and became the basis for the engineers to start working on something sophisticated enough that lead to Skynet.Time travel is so hysterically unrealistic and problematic in the first place, I don't see how a minor detail like something mentioned in a sentence in the first movie can affect the otherwise 'complicatedness' of time travel to a sufficient degree to be worth arguing.
And considering the leap in technology from the t-800 to the t-1000 there is a chance that they also had the time to make changes to the time machine.
Seriously, if we are to take a BTTF (which as far as I know is the most 'correct' time travel movie) as an example, Skynet would know the second Arnold was sent back in the first movie that they had failed. If they hadn't failed the 'future' would be changed instantaneously (from Skynet's perspective) Same for the second movie. Unless there is an alternate universe created for every space-time 'paradox', which would make it completely pointless.
T2 is still one of my favourite movies though
(alongside Mad Max 2 in which people use muscle cars to fight each other over fuel ,in a world almost void of fuel, do I like unrealistic plots?)
Here's another problem. If time travel in the "Terminator" universe is one-way, how did they know that nothing metal could go through?
Skynet sent stuff through time and saw what happened. A day back would be enough.