STAYING ON THE BALLOT: BALLOT CHALLENGES
In states where it is allowed, it is standard practice in politics to challenge your opponent's petition signatures in an attempt to get them below the minimum needed to appear on the ballot. Why face them in an election if you can prevent them from ever appearing on the ballot? Reasons for challenges include signatures that don't match up with the one on file, incomplete addresses, ineligible voters, forms not properly signed or dated by the circulator and more. In 1996, on his first State Senate campaign, Barack Obama challenged the legitimacy of his opponent's petition signatures. All 4 of his opponents were removed from the ballot and Obama won with no opposition.
As a candidate, you should plan on having money in your budget for an attorney to fight any challenges. Best case scenario, you don't get challenged and have extra money in your budget. Make sure you hire a competent election lawyer to help you with this process. I had City Council candidate in a 3 way race. Everyone challenged everyone else. Each needed 750 valid signatures. The independent candidate only submitted 778 signatures and the lawyers quickly discovered a page that the signature gatherers had not dated and initialed. The 30 signatures on the page were disqualified and as soon as the Independent was below 750 valid signatures he was thrown off the ballot. Then there were only two candidates left. My candidate stopped his challenge of her petitions because we couldn't get her below 1000 signatures and it wasn't worth the money to bring in a forensic handwriting expert to challenge individual signatures (yes, campaigns actually do this!)
The time came to examine my candidate's signatures...He had gotten 1150 signatures, but had gone through on his own and struck 200 that were invalid, so he claimed he submitted 950 signatures. If the opposing lawyers successful removed 200 names he'd be off the ballot. The lawyers challenged 215 names. What they didn't realize was that from the perspective of the law, my candidate had turned in 1150 LINES OF SIGNATURE. The ones he struck himself still counted to the total number. The opposing lawyer should have listed the line numbers of the signatures my candidate had removed - they were freebees, but he did not. The judge said "He's got 1150 lines. Even if I give you every one of your 215 challenges, he's above 750. Case dismissed."
As a general rule, you should seek double the signatures actually needed to ensure that you have enough valid ones, since many people will sign who are technically ineligible to do so (don't live in the district, not registered to vote, etc.) However, the numbers alone are not sufficient to guarantee you make it. In 2012, a State Representative from a major city submitted over 1,600 signatures to the election board. He only needed 300 valid signatures to appear on the ballot, but instead of circulating petitions within the district himself he passed his petitions around a few large churches.
There are a few problems here. First, by circulating petitions in a church the candidate had jeopardized their tax-exempt status. Second, the churches were outside of the district and most of those who signed were ineligible to do so. His opponent challenged his petitions and upon examination more than 1,300 were thrown out. He didn't have 300 valid signatures, so in spite of being the incumbent he could not appear on the ballot.
Know the rules, get more signatures than you think you need and have a good lawyer.