These shifters are troublesome, I have encountered this problem on several of Shimanho's triple-capable shifters.

I always do the same thing to minimize the chances of this happening.

First, the chainrings checked for trueness, then I adjust the hi-limit screw as tight as is permissible w/o having any rubbing in the highest gear.

Next, I check cable/housing condition and the screw (if any) that secures the cable housing stop to the downtube, and lube cable and the bb cable guide if needed.

The last step is to get the cable adjusted properly so that the shifts to and from the middle ring are "solid", then make sure the cable is fully taut when the big-ring is selected. This last step is the most important, as it helps to prevent the lever from accessing the un-needed "phantom" shift position that exists out beyond the proper "big-ring" position. That's right, there should be NO derailer cage movement visible when attempting to shift beyond the proper "big-ring" position, that's the kind of springy, back-and-forth cage movement known as "over-shift", and is what may allow the shifter to lock up in an overly-tensed (cable) state which then won't allow the mechanism to downshift.

Having adjusted this way, the only thing left to address is to maintain (religiously) the cable tension adjustment by regularly taking up ANY slack that might allow the aforementioned "over-shift" movement. The high-gear limit screw should not need future maintenance beyond every several thousand miles, but that cable should be re-tensioned as necessary on a somewhat regular basis by turning the adjusting barrel a quarter-turn every now and then as required.

It will still be possible to lock up the shifter by FORCEFULLY upshifting beyond the proper big-ring position (through the sheer force of stretching the thin cable wire), but it shouldn't happen when riding unless you're a weight-lifter or an extremely aggressive shifter.

Note that the "phantom" click is there allow a double-ring crankset to use the same shifter as a triple-ring setup AND to account for the diversity in cranksets and chains (older designs, compacts and other's touring designs in particular) which might REQUIRE the derailer to move a bit further to effect the shift to the big ring.
This is apparently the price that is paid for allowing "compatibility" with cranksets which do not conform to exact Shimano standards, but I've seen all-Shimano double-ringed bikes do this when the adjustments are allowed to stray. The adjustable-reach R700 shifters are often used on women-specific models on otherwise-double equipped bikes, and can suffer this problem if not adjusted every one or two thousand miles.

I would also point out that cables in poor condition and contaminated housing liners can introduce enough springyness and friction to make proper adjustment a vague process which invites too-easily-accessed shifting out to the troublesome phantom click that's buit into these shifters.
I have had a go at fixing quite a few of these out on the road, and usually have to start by somewhat forcibly over-shifting the lever briefly, followed by an immediate stab at the downshift lever. Ugly, but it usually works without having to un-secure the cable or housing stop screw in order to free the taut cable.

This problem can be kept at bay!