Corporate actions are processed by the transfer-agent and dtcc and land in your account overnight. Mandatory actions such as a stock-split or merger happen to you automatically. Voluntary actions such as a tender-offer or a rights-issue election require you to respond by a deadline, and brokers set their own cut-offs a day or two before the official one.
For traders the risks are practical: option contracts get adjusted, historical charts are restated, resting orders may be cancelled, and short positions can be recalled.
Example: a 1-for-8 reverse-split converts 4,000 shares at $0.50 into 500 shares at $4.00. The position value is unchanged at $2,000, but the stock is now marginable and the options chain is re-struck.
Related: stock-split, tender-offer, dtcc