A nonzero percent error just means two numbers disagree. It doesn't say why — and jumping straight to "the experiment must have been sloppy" is one of the most common wrong turns in a lab report.
Every intro lab report ends with the same ritual: calculate the percent error between what theory predicted and what you actually measured, then explain it. Most students reach for the same explanation regardless of the number in front of them — "human error," "we must have measured wrong," "our technique wasn't careful enough." Sometimes that's true. But a percent error is nothing more than a distance between two numbers, expressed as a percentage. It carries no information at all about why those numbers differ — and treating every gap as evidence of a mistake skips the actual scientific question, which is figuring out which of several genuinely different explanations is really responsible.