Most losses in matched betting do not come from anything unpredictable, they come from a small, repeatable set of avoidable mistakes. Here is what actually trips people up most often.
Sizing the cover bet by feel instead of by calculation
The most common mistake by far. The correct cover stake depends precisely on the odds available on both sides at that exact moment, and eyeballing it "close enough" quietly gives back profit even when it does not lose money outright. A cover sized even a little off does not usually blow up the bet, it just erodes how much of the bonus''s real value you actually keep.