Smart contracts have an unusual property that makes them unlike almost any other kind of software: once deployed, the bug is in production forever. Web2 services can patch a vulnerability and roll out a fix in an hour. A smart contract that's been deployed and is holding value can't be quietly patched. You can sometimes migrate to a new contract, sometimes deploy a workaround, sometimes pause the contract while you figure it out. Often you just take the loss. The bug is public; the funds were too.
This is the reason smart contract audits exist as a category of their own. Code audits in Web2 are useful; code audits in Web3 are mandatory for anything holding meaningful value. The good firms — OpenZeppelin, Trail of Bits, ConsenSys Diligence, Code4rena's competitive audit format, and a handful of others — have specific expertise in the failure modes that Solidity (and other smart-contract languages) produce. Reentrancy attacks. Integer overflow patterns. Front-running by validators. Flash loan composability. The kinds of bugs that aren't bugs in Web2 because the underlying assumptions don't hold.
What a real audit looks like: two to six weeks of dedicated review by senior engineers who specialize in this work. Cost ranges from $25K for a small contract to $250K+ for a complex DeFi protocol. The deliverable is a report with prioritized findings — critical, high, medium, low, informational. The good firms also test their fixes by reviewing the patched code, not just the original. A single-pass audit without a re-review of the fix is half an audit.
What teams try to skip — and shouldn't — is the time. Six weeks is a long time when you're trying to launch. The temptation to use a one-week "quick audit" or a fully-automated tool is real, and it's exactly the temptation that produces the post-mortems we read about three months later. The cost of doing the audit right is real; the cost of skipping it can be your whole protocol. The math, when written down, is not close.