Deliberate mistake: As usual when the Hulk reverts back into Bruce Banner, he is left wearing nothing but tattered pants. When Mitch McCutcheon sneaks Bruce Banner into the nuclear research facility, Banner is shown wearing his usual outfit of a gray collared shirt tucked into purple slacks that are intact. Given that the Hulk was just in the middle of the Nevada desert, Bruce Banner would be in no position to have a change of clothes on him, and the idea that the top secret facility would have an exact match of his go-to outfit is ludicrous.
Man to Man, Beast to Beast - S1-E6
Deliberate mistake: When Bruce Banner is crawling away from the town that Hulk attacked, he passes out in the snow and is rescued by Sasquatch/Walter Langkowski. When he awakens in Langkowski's lab, he is wearing his usual outfit. Since he was only wearing tattered pants when he passed out, that means his clothes were given to him by Langkowski, which is extremely unlikely since the two hadn't seen each other in a long time and Langkowski didn't even know Bruce was looking for him.
Answer: The Hulk is a giant brute who smashes everything in his path when he is angry. In Ang Lee's "Hulk," General Ross, played by Sam Elliot, explains it to his daughter.