Imperforated (yes) Error (no!)
During the late nineteen eighties (1987) Royal Mail experimented with with the way in which some of their bar code booklets were manufactured. As a result of this, for a period of about three years some completed booklets were trimmed or guillotined after the assembly, resulting in several new stamps issued during these times having straight edges instead of perforations on all sides.
As the experiment went on for three years or so, there are many straight edged single values to collect, including double headed anniversary stamps, which were issued to commemorate the 150th anniversary of the penny black in 1990.
The complete list of stamps issued runs into many variations of different values, including 1st class, second class and numerical values, and different colours of the same values with bright or dull papers papers. The list is to long to name them all here, but I can show you a complete pane which will give you an idea of what to look out for.
These stamps are often overlooked by philatelists when they are forming a Machin collection, as they are only mentioned ( listed) in specialised catalogues. All the varieties of straight edged stamps are collectable and some of the rarer types fetch very good resale prices.
The booklet pane below right originaly cost £1.56 on the day of issue, today (as singles ) it is catalogued perhaps 10 x or more of the initial face value.
What to look for
You will notice that there are four different stamps in the make up of this pane.
Stamp one has a straight edge to the top, stamp in position two is cut both on the top and right edge, stamp three, just on the bottom and stamp four has straight edges right and bottom.
Booklets are still readily available on auction sites if you are prepared search for them and pay the premium, as are some of the singles both used and mint.
First day covers on the other hand showing these issues are getting harder to find, so if you come across any, tuck them away for that rainy day, you will not regret it.
Keep em peeled