Factual error: When Biff and Marty discuss the Almanac in the Alternate 1985, a magazine is visible. This is the March 1989 issue of "Mustang." (00:57:05)
Factual error: When Marty enters the 1955 Hill Valley, one of the cars has a white licence plate with blue text and a sunset, which was not used until 1982. (01:04:54)
Factual error: Marty is pursuing Biff at the high school as the other Marty pulls up with Lorraine in Doc's car. In the parking lot, the car closest to Biff is a brown 1957 Chevrolet. The front grill and styling are distinct and unique to the 1957 model year. In November 1955, it would be plausible to see a 56 Chevy in the lot, but a '57 is a bit early. (01:16:42)
Factual error: In 1955, Doc says "Sunrise should be in around 22 minutes." This wouldn't be possible as the sky is fully dark (except for a slight bit of light on the other side). If only 22 min from sunrise, even on a stormy overcast morning, the sky would be fairly light by then.
Answer: At the end of the first film, Doc says, "what the hell" in response to the letter Marty gave him. So this shows it is not that strong a conviction, especially in the face of definite knowledge about something bad happening. It is also a way for Doc to repay Marty.
MasterOfAll