With the S&P 500 up by about 16% over the past 12 months, but the COVID-19 pandemic still ongoing, many investors fear another market crash is imminent. To be clear, stock market corrections -- rapid, double-digit-percentage price declines -- can't be predicted with any accuracy. They're more common than many think, though. Broad market declines of 10% or more occur every 12 to 18 months on average, but prices usually fully recover within a few months. Those short-term fluctuations can offer great buying opportunities to long-term investors. We asked three Fool contributors which stocks were on their downturn watch lists. They suggested that Intuitive Surgical (NASDAQ:ISRG), Roku (NASDAQ:ROKU), and CrowdStrike Holdings (NASDAQ:CRWD) would be good buys now, but would be even better to pick up if the market takes a sharp turn south. A pioneer of next-gen healthcare technology Nicholas Rossolillo (Intuitive Surgical): I think the healthcare industry is ripe for technological disruption in the next decade, and Intuitive Surgical is one of the best ways to play that coming trend. The pioneer of robot-assisted surgery is making a strong comeback after a rough 2020, and it's poised to sustain its momentum for the foreseeable future. The scheduling of surgeries -- especially elective surgeries -- was disrupted significantly by the pandemic. This was still a problem for the company during the fourth quarter, when fresh restrictions were imposed in some parts of the country to try to stem the rapidly surging spread of COVID-19. Intuitive said the number of procedures using its da Vinci surgery system increased only 6% year over year in Q4. That was, however, a big improvement from the 19% year-over-year decline it reported last spring. And though the number of new da Vinci systems shipped in Q4 was 3% lower than a year prior, the company ended 2020 with an installed system base of 5,989 worldwide -- up 7% from the end of 2019. The end result was that its full-year revenue and net income declined by 3% and 13%, respectively. Not bad, given how terrible a year we just had. The increase in the number of da Vinci machines installed suggests that Intuitive should quickly return to growth mode this year. The company also remains highly profitable (net income was $1.06 billion, with a profit margin of 24%), giving it the financial flexibility to keep developing new procedures for its robotic surgery systems and developing new systems for use in the medical field. Meanwhile, the company is set to start lapping the depressed financial results from a year ago -- teeing it up to display dramatic year-over-year increases in revenue and earnings. Granted, Intuitive Surgical's stock is expensive. It's currently trading for 81 times trailing 12-month free cash flow. But given that the company is among the leaders in disrupting the healthcare status quo and driving better patient outcomes, there are good reasons for it to trade at a premium. For investors looking for a solid investment in healthcare that they can hold for the next decade and beyond, I think it's a solid buy right now -- and would be an even better long-term deal for investors to pick up if the shares took a steep tumble. Image source: Getty Images. I can't wait for a juicy Roku discount Anders Bylund (Roku): Entertainment technology developer Roku is one of my favorite long-term bets. It's a winner in the media-streaming market, having established itself as the go-to choice when TV makers need a media-management solution for their latest living room centerpieces. The Roku-branded set-top boxes and HDMI sticks were a nice start, but the real money is in the software licensing market. That's an incredibly scalable business with generous profit margins, and Roku gets to ride the coattails of content-creation titans such as Netflix (NASDAQ:NFLX) and Disney (NYSE:DIS) all the way to the top. The market has caught on to Roku's massive business opportunity, driving share prices 260% higher over the last 52 weeks. I still think it's a solid buy at today's prices, especially if you are building your investment positions with a disciplined schedule of small buys over time. And if the stock suddenly drops 20% or more due to a general market calamity, I would be first in line to boost my Roku holdings. This vigilant cybersecurity play should be at the top of your watch list Billy Duberstein (CrowdStrike): A big market pullback is a great opportunity to upgrade the quality of your portfolio and buy long-term market leaders. That's why I would keep my eye on CrowdStrike, which is currently disrupting the high-growth cybersecurity market. Its endpoint security product was built with a cloud-first, AI-based architecture ideal for today's enterprise environment. Its lightweight Falcon agent can be attached to any endpoint, whether a cloud server, laptop, phone, or corporate server. Every Falcon agent then reports back to CrowdStrike's centralized Threat Graph, which uses AI to continuously tweak its algorithms and respond to new threats in real time. CrowdStrike has become a clear leader and disruptor in the endpoint security market, which is still fragmented and has a long way to go. Though it has been more than doubling its customer count annually for several years already, CrowdStrike still managed to grow its customer count by 85% year-over-year last quarter, and boost its subscription revenue by 88% over the previous nine months. Moreover, management is working on new "modules" -- right now, there are 11 -- offering it opportunities to upsell those clients on complementary services beyond endpoint security. Back in September, CrowdStrike bought Preempt Security, a leader in identity management. And then last week, it made a bigger and bolder acquisition -- security start-up Humio, a leader in something called XDR, or eXtended Detection and Response. XDR tackles the problem of having too much data coming at customers all at once. Bringing Humio's products into its system should enable CrowdStrike to better filter out those masses of unnecessary data and deliver more useful, actionable insights to its clients. The unfortunate Solarwinds (NYSE:SWI) hack that was revealed in early December put an even greater focus on cybersecurity broadly. In response, Solarwinds actually chose CrowdStrike to fix its problem and guard its network. Unsurprisingly, the company's stock soared in the aftermath of that news -- and it had already gone up a lot prior to that in 2020. The company currently trades at a whopping 68 times sales, which is an incredibly expensive valuation. That being said, CrowdStrike is a best-in-class disruptor with high gross margins and a huge tailwind, so a premium valuation is somewhat warranted. Therefore, investors should look to scoop up this premier cybersecurity play on any large market pullback that takes CrowdStrike's share price down with it.