There are a number of different errors, so it's tough to say: unwanted clip snippets, gaps, jump cuts, misplaced overlays. Following the suggestions above should minimize a recurrence.
There are a number of scenarios that can produce unexpected results: if a new clip is inserted into an old (not at the join with an adjacent clip) it will split the existing clip. That may not be readily apparent when the timeline is compressed.
When inserting, keep a close eye on the timeline. If you make a mistake immediately press <ctrl-Z> (EDIT | UNDO) to go back, step-by-step. Or reload the most recent back up, if appropriate.
Always review the exports, to be sure that embedded errors - not apparent on the timeline - aren't causing issues. That's a good reason to segment long projects, as each part can be checked.