Professional Indemnity Insurance (PI) NOT PL, is a potential can of worms from a liability perspective.
The bigger problem as both Geoff and DJP have alluded is that if a claimant or claimant's lawyer gets wind the designer may be at fault and the designer has assets, they will go the designer or the one with money before any liability is even established. It is not the 'at fault' liability that is the immediate problem. The immediate problem is the cost to run a defense. You need a lot of cash to run a defense if you are NOT insured.
If you are insured, your costs may be covered, but you have no control over the outcome. Your insurer steps in and will make whatever deal they see fit on financial grounds to settle, sometimes even if the complaint is defensible, so your reputation is at risk and you have no say in the settlement. Once they insure you, they own the whole process; so even if your design is OK, once litigation is commenced it is a potential nightmare.
PI insurance covers you on a 'claims made' basis. What that means is the year of the alleged design flaw is not so relevant (obviously you need to have been insured at that time), but you are not covered if your insurance is not current in the year the claim is made. You get in a situation where you have to maintain permanent PI insurance and usually with the same insurer. When approaching retirement you usually do a deal on a diminishing premium basis, but it must be continued past your retirement.